as you want to do all tasks by your own, I can suggest you to make some clear concepts of what you want to make and define your gameplay, then model assets, test them in editor and script for those which need to have an interaction (don't forget your concept). keep on modeling and test them in a little scene,(don't forget your concept!), after you modeled one of your levels(even a little level), you can start terraining and place all your models according to your concepts. now you can bother with AI, GUI, and so on..