How to plan a application/game/whatever

I have been strugling for months to finish a simple missile comando game in my free time, with no sucess.Very oftem i make a class, and later want to add some more to the game, and have to go back and remake the first class, so i was thinking that some problems like that may be because of the lack of a plan…
So,is there any sugestion from those of you from the programming world about how to plan a game/application? I dont know, i dont have a formal education on that,im from the ‘design’ world, (and since AS3 is not for designers and i know as3 what i am now?)
I’ve heard of uml diagrams, they seems just a list of public variables and methods from a class, why it dont have events?They are not important when planning a game?
Thanks in advance