I was just getting into the fandom, after finishing the first season of MLP. I had let my voice go for a year, but was kinda inspired by the creativity in the fandom to get back to doing what I love- which is singing and acting. I'd made my MEMJ0123 youtube account when I was 13, in 2007, and I just dug it up to upload my pony videos on. So I was mainly just auditioning for pony fan projects, which was incidentally how I got involved with SFS. IRL I was in my first year at University, not knowing what the heck I was doing with my life!
Well, they all had their own challenges. I can't really say which was the most challenging. Some characters were difficult to voice pitch-wise, some were difficult acting-wise.
For Voiceover, start out simple. Audacity is a great free audio-editing program. XLR mics will produce better quality sound, but they are more expensive, so I recommend starting with a USB mic. I started with a Samson C01U, it was great. Get a pop-filter, too. Also making sure you have a good recording environment will increase the quality of your sound immensely. If your space has a lot of echo, you can prop up a blanket around your recording station. Try to be aware of the background noises around you, like cars going by, people walking around, other noises, that kinda thing.
Beyond technical stuff, all sorts of different funny voices are great, but you really want to focus on the acting part. Those funny voices won't mean anything if you can't actually act with them. Take improv classes, theatre, even singing lessons help (especially with tone and range of pitch). Have fun with your voice before you try to make a demo reel. A demo reel won't be useful to you until you have an established vocal strength (like, do you do deep voices best? High voices? Mom characters? Cheerful characters?), as the reel will only contain the voices you do best. Fandom stuff is a great way to learn things about your voice. Get involved in group projects, get to know people, make friends! Connections will go a long way, so make sure you are always professional. Stay away from drama if you can help it. The internet can be a dramatic place
I think that's about it for now! Hope that helps a bit! Good luck!