>955
It's called a dakuten.
It's a voicing marker. For characters that start with a plosive consonant, the consonant becomes voiced, and for everything else, a B is added. That's the gist. Rule of thumb: if there's a "soft" version of the consonant, that's usually what it turns into.
So か ka becomes が ga, た ta becomes だ da, etc; へ he becomes べ be and ほ ho becomes ぼ bo.
There are some complications, though; for example ふ fu with a dakuten ぶ is not vu but bu. If you know what voiceless plosive consonants are you'll probably be able to see the regularities, but otherwise I think you're better off just memorising which become "soft" versions and which get a B instead.