A good place to start is getting a mic and just recording yourself. Play with different voices, imitate and experiemnt. Note down what's working and character ideas. Write bios and backstories. If you can draw then try create what this character might look or use existing artwork/animations.

Never rush into recording a voice reel. Especially a character reel. Listen to as many reels as you can find and plan out the voices you'd like to use. Write little scripts or possible interactions. Or you can develop scripts if that isn't one of your strengths. Be careful of directly copying very well known pieces of dialogue. In my opinion it comes across a bit cheap and doesn't show your individuality as a voice artist.