A development plan is very important.
Start with the stuff you only need for the very basic working version. Then add the other stuff in steps. Like:
Step 1: Load Level, Movement, Save
Step 2: Enemies, Fight
Step 3: Make Levels, Replace guard.mdl with something fitting.
Just as example. Whenever great things come to you in mind, write them down and look in what future steps they can be included. Be sure you can handle the active step in time.

I do it that way and it works very well for me.

Oh, and you probably have to rewrite your code once or twice while you learn. ^^

edit:// Was maybe not clear in my text, but like Pappenheimer said write everything down and stick to it. And the AUMs are really helpful. In the intervies especially the last point which is often "What do you want to say to beginners" or something.