You should be fine with a 32GB card recording at 1080P @ 30FPS. Just make sure you pick up a class 10 card with 45MB/s write speed. That way you can use the same card in a new Hero 3+ or even later camera without any issues if you ever get one.
Yep, spot on advise above. 32GB is perfect. 64GB is only really needed if you're doing the 60fps of the Black. Even if your on a strict budget a 16GB is actually decent amount, particularly if you're doing a day or weekend type footage (and not filming every single run mind you), does the job. ie the battery will die before you fill the card.
Also make sure you order Micro SD card too, not standard SD card...since you mentioned your new to it all just thought worth adding to the post above. Hero 3 family made the shift to Micro SD, from SD.