First, please try to avoid doublers, pitch correction, choir and harmonizer effects. These are the scourge of modern music production; they may feel reassuring and initially impressive, but the effect wears out quickly and ultimately you'll just sound like everybody else who can't sing.
(Disclaimer: if your aim is to make commercially successful records, my advice may not be the best. But but hopefully you also care about not ruining music...)
Of course, the most important thing to a good vocal sound is the voice. It doesn't need to fat, it doesn't need to be pitch perfect, it doesn't need to be, erm, good, to be... good. But it should be you. Try to find the register where your voice works best. Keep experimenting, be it with acoustic guitar or with a band. If you always find just the right expression and character, then it shouldn't matter as much whether your voice itself is thin.
That said, if your voice is simply too weak to be made audible properly, in the context you want to sing in, then some electronic assistance is necessary and probably also not problematic.
First, you need a PA that's loud† and clear, without any nasty resonances. Acoustic-guitar amps can sound ok for this purpose, at least for monitoring purposes... but they're scarcely really sufficient as a PA replacement. A couple of semi-decent active speakers will definitely serve you better.
Loudness brings some new problems with it:
- Feedback is always a risk live; the louder you amplify, the more. Generally, the more linear your PA's response, the less feedback you'll get. Beyond that, feedback destroyers can help a bit.
- Some parts of the voice, you probably do not want that loud. In particular,
- Boosting the bass range is usually not a good idea, because it results in a boomy, undefined sound. In fact, with a good PA you'll probably want to cut the low end. For that and a lot of other tweaks, you should have a good equaliser at hand. Modern digital consoles come with good fully parametric eqs, those are ideal. Graphic equalisers can also be useful, but they're also often used badly.
- Some peaks may come out too loud, and either overburden the PA or just sound annoying. Well, that's what compression fixes. You may think this doesn't add much to the voice, but that's because a good compressor acts very subtly. In fact, a compressor can make a voice sound a heck lot better in a mix, while still staying very faithful to the voice itself.
Apart from that, there's one thing that can significantly fatten up voice in a very useful way: smooth nonlinear saturation. This is best known from traditional analogue recording equipment, mostly tape machines and tube preamps. The latter are still a really nifty thing to use, even live.
Finally, reverb and delay. You'll always want at least a little bit reverb, but don't overdo it: if there's too much reverb, the voice will in fact start to sound more distant and feeble!
And delay is really nice as a space effect, to highlight some song parts. But if you're always followed by an echo, it'll seem a bit ridiculous.