reddit 'overwatchuniversity' has some good guides and generally helpful folks.
Generally speaking though - just play. You can only play 'quickplay' (the 'practice' mode) and I think arcade (1v1, 3v3, random heroes etc) for the first 25 player levels anyway, so any mistakes you make won't affect your overall ranking.
There's an in-game testing range if you want to give them a whirl before trying against humans, or you can play vs AI games for a bit if you want. The in-game ability summaries for most characters are pretty much bang on though.
If you want something more in-depth, check out furiouspaul's website for a variety of guides.