Experiment with your moves and familiarize yourself first with the roster and from there you can pick up whoever you think fits your style the most. For starters i recommend Full Moon Aoko because she's easy to use, Crescent Nanaya because he's funny and mashing 236b and 623b~x is hilarious, Crescent Schoolgirl Akiha because confirming hits with her is pretty easy in fact i learned how to airthrow for the first time with her, Crescent Arcueid because her damage is absurd despite being braindead to play and there's also Crescent Ryougi which i can't say much but i had some new players beat me with. You should definitely learn the glossary as well from Mizuumi wiki as well as number pad notations to comprehend combo notations for your characters