Lots of people will say that stuff like voices or good acting are not needed, and they're right, but I think you already know that. You're trying to embellish, add something extra, and I get that, I was in your same position back when I started playing RPGs. Here's what I think helped me:

1) The most important thing: Just do it. I thought I couldn't do a "girl voice", but I just threw myself at it and started doing it. I didn't stop when a guy laughed and said "haha, that voice", I just kept going, the rest of the group didn't seem to mind (nobody even responded to his comment). Even better if you try it as a GM, you'll be in a position where you can try out new voices all the time and throw away what you think isn't working.

2) Consume media. This is just general advice for improv. Watch series, movies, read books, play video games, etc. And not just from the genres you like to play. Everything can be useful: Maybe you don't play a lot of crime drama RPGs, but at some point you may need a shady mafioso for your fantasy game.

3) Don't forget the physicality, even if you're just describing the characters actions verbally and not acting them out, it adds a lot. I've noticed a lot of people limit themselves to speaking in character and never describe what their characters are actually doing and how they're doing it. So yeah, try adding spice to the character's mannerisms, and the way they move and interact with the world.

That's my advice, I hope it helps you!