1 slight knowledge of music
Let's get started! This is a lot easier if you are a girl or a guy with a particularly high singing voice, but do not be discouraged. It is rather easy to get to do this.
Step 1: Define your voice type.
I suggest this only for sopranos, tenors and altos so that you don't hurt your voice. I myself are a middle soprano, so I can do all of the ponies, with the exception of Rainbow Dash and Pinkie Pie. My voice just doesn't do that. Here are a few suggestions of voice types for ponies.
Fluttershy- Light and airy Mid Soprano
Dashie- Guy-like Alto/tenor
Rarity- Pompous High Soprano
Twilight Sparkle:Brainy Alto
Applejack- Country slur Mid to low soprano
Pinkie Pie- Energized VERY high soprano.
Pick your pony accordingly.
Step 2: Find a few starter lines.
Rainbow dash: I can do that in ten seconds flat!
Applejack: Now what the hay did you do that for?
Pinkie pie: Isn't it GREAT???
Twilight Sparkle: Dear Princess Celestia...
Rarity: Oh my stars, darling!
And there you go! Try them out and contort your voice to match the character's by adding a country slur, a british-y accent, going up about half a pitch, down half a pitch, you should know .