Can you attack again after I decide not to block?
No, you can only attack once per turn

There are certain cards that give you a second attack, but normally, one attack per turn.

If you are new to Magic, you should have a look at the Basic Rulebook (pdf) on the Magic rules website. It contains all the essential info to get you started.

If you encounter situations not quite covered there, you can always check back here, or try to figure it out yourself in the Comprehensive Rulebook (don't use that one to start out).