One aspect of DnD which seems to strike the most fear into the hearts of would-be DMs is the difficulty of giving your NPCs fun and distinctive voices. We watch professional DMs switching effortlessly between different accents, and fear that for our NPCs to be loveable and memorable that each needs a unique voice. First of all, this is simply not true. I've played in amazing campaigns with DMs that have never switched from their own voice, and absolutely loved every minute of them. If you're not comfortable modulating your voice, then feel no obligation to do so. It does not make your story boring or your sessions uninteresting. Seriously, don't sweat it. But if you did want to start experimenting with a few different voices for your NPCs, then it can open up fun and interesting opportunities to your campaigns, and in my experience is one of the quickest and easiest ways to get your players invested in an NPC and seek them out in future sessions. With that in mind, here's a little bit of advice and a few tips if you want to go down this route.
Voices do not equal accents
I cannot stress this enough. A unique voice for an NPC does not mean you have to learn a completely new accent! If you can do them, great! But if not, learning an accent convincingly can be a massive undertaking. This one thing can easily be as much of an investment of time as the whole rest of the prep for your campaign. And then what if your players just decide they're not interested in that character, and walk on by? Probably not the best use of your time, right? Accents also have the disadvantage of everybody having a vague idea in their head of how it's supposed to sound - so it's an easy way to increase your anxiety as a DM, as you get wound up in worries about how convincing other people are finding your accent, and fear that it's gonna slip any minute. Thankfully, there is a better way.
Keep it simple, stupid
Instead of going all in on learning a new accent, let's start with just very simple modulation of your voice. Try speaking at a slower cadence than you would normally. Imagine you're doing a voice-over for a nature documentary, and leave a number of dramatic pauses here and there. Next, try increasing the cadence of your voice. Speak as fast as you can, as if you're always in a desperate hurry. Next you can try to vary the pitch of your voice - try speaking at a higher or lower pitch. After that, you can try talking in a whisper, or with a raspy voice. Now try speaking in a gruff angry way, or in an airy and floaty manner. None of these things take a great deal of effort - they're probably things you've done at some stage in your life when impersonating somebody - well, now you can use them to impersonate your NPCs.
What's great though, is that by mixing and matching these characteristics, you can start to make a variety of different voices. Perhaps one speaks in a high pitch but very slowly. Another speaks in a whisper with a fast cadence. Yet another has a low and gruff voice, as if he's perpetually angry. You get the idea. Honestly, with very little difficulty, you can come up with perhaps ten slightly different voices. Obviously, you'll likely have more than 10 NPCs in your campaign - but even with this minimal effort, you're well on your way to giving your NPCs their own voices. And with time, you can then elaborate on these voices, adding in other elements as and when you feel comfortable doing so.
Don't worry if it's awful
In a campaign I ran a couple of years ago, I had a character who was basically Sean Connery's character from The Rock. And, of course, to complete the illusion I absolutely had to give him a Sean Connery accent. Usually I can do a passable impression out of game, but within game where I was constantly switching from his voice to my own (or indeed to different voices for other NPCs) I found my Sean Connery accent kept slipping and myself becoming increasingly self conscious. At one particularly frustrating point when I didn't know who the heck I sounded like, I said to my players ah, you know what, I'm just gonna speak in my normal voice for him. There was uproar. My players told me that they loved the character, and really wanted me to continue with his voice - and it was then that I realised that any time they were laughing when I was doing the accent, it was because of how much fun they were having with him, and not laughing at me for my feeble attempts to accurately emulate Sean Connery's voice. The accent stayed, and I learnt a valuable lesson: people are never judging you as much as you are judging yourself. And if you're making an effort in-game to create something they'll enjoy, they will respond to that. Don't sweat it if it's not prefect. It's a game, not a performance - so if you're all having fun, that's all that matters.
Practice makes... better
Everything gets easier with practice - and voices are no different. If you're in any way easily embarrassed, though, then I suggest trying them out in private. I usually practice my NPC voices while I'm in the shower, or driving, or alone on a bike ride. Anywhere where my partner won't hear me, to be honest. There's something quite fun about asking yourself what a bullywug would sound like, and then trying to have a conversation with yourself in that voice. And again, these don't need to be perfect - but anything distinctive and memorable would make a great addition to your repertoire. One thing that I find helps is starting off by reading something in your target voice (maybe don't get a book out in the shower or while you're driving, though). By having what you need to say in front of you, you don't have to spend any mental energy firguring out what you want to say, and you can concentrate solely on getting your mouth, tongue and vocal chords to do what you want them to. With time, as the voice becomes easier for you, you won't need any prompts for what to say, and you can just use it as you would your own.
Get in their headspace
By trying to embody the headspace of the character, I find it so much easier to stick with their voice and not keep sliding back to my own. This doesn't have to be Daniel Day-Lewis method acting level commitment - simply knowing a couple of things about their background and aspirations will be really helpful. It will again free up a bit of mental load when roleplaying - making answering any questions your players ask of them easier, and giving you just a little more headspace to stick with the voice. Adopting any gestures or turns of phrase you think the NPC would use will also help you to remain in character easier.
Not every NPC needs their own voice
Please take this to heart. Not every NPC needs their own voice. It's is prefectly fine for some (even the vast majority) of your NPCs to just use your own voice. Even professional DMs frequently use their own voices for NPCs. I've been watching a lot of Dimension 20 recently, and even though Brennan Lee Mulligan has an incredible repertoire of different NPC voices, some of the time he just uses his own voice for them. And damn, if it's good enough for him, then it's certainly good enough for the rest of us mortals. And by only having some characters with unique voices, you make those NPCs more special and memorable, and help them to stand out.
So, really, it's up to you how far you go with this. If you have a real knack for voices and accents and impressions, and are dead-set on giving each NPC in your campaign their own voice, then absolutely go for it! But if not, then dont worry about it! If you stick with just your own voice throughout the campaign, you can still make something incredible. And then if you want to sprinkle one or two different voices in amongst that, it can really bring your world to life, and encourage your players to connect more deeply with it. Most importantly, try to have fun while you're doing it. Your players will certainly appreciate your efforts - so you might as well have some fun with it as well!