The first few times playing can be a bit confusing with all the choices. When you get cards during charity you don't have to do anything right away. You can save them until your turn.
If there are several people playing you could use the cards you got during charity to affect the battles with the monsters. But you don't have to.
You could also just hold them and wait for your turn.
After all your actions are over on your turn, if you are the lowest level, and you have more than 5 cards, you just discard down to 5.
There are a couple of great demo videos, one on the sjgames site here:
http://www.worldofmunchkin.com/game/demo.html
Another can be found searching YouTube