Can someone help me with how credit cards work?

Currently my score is in the 640’s, I’m really trying to get above 700 (I’m 21). I have two credit cards with no debt, I pay off the balance every month. My score went down one month because I spent over 50% the limit, but I paid it back in full by my payment date. What percentage of my limit should I be using? Should I always stay below a certain percentage? Does it matter if I make one big payment a month or splitting it up? Thanks! :)