I use Squadron Green putty.  They also make white putty.  A lot of people use super glue, but I have trouble knowing when it's dry enough to sand vs. when it's dried for too long and can't sand it effectively.  Start trying the Tamiya putty.  If you don't like it, try something else. 

A good source for modeling is Youtube.  Search "Tamiya putty" or "rescribing" and you'll get informative how-to videos to help shallow the learning curve!

Aaron