I’m totally new to zbrush and I know just the basics. I’m looking for a alien tutorial for beginners. I must understand what I’m doing and it has to cover stuff as modeling, sculpting, rigging, etc. I’ve searched all over internet but I can’t find one! PLease help!!! 
The problem is that you’re looking for an alien tutorial. You should be looking at tutorials that teach you how to sculpt, texture, etc. Once you understand the basics, you can create anything you imagine. If you don’t understand these fundamentals, it won’t matter what you’re trying to create – you won’t get very far.
As for aliens, it’s highly unlikely that you’ll ever find a tutorial about who to create one. The reason for this is that an alien can be anything you imagine. Aliens and monsters are generally the easiest things to create because you have very little in terms of real-world limitations to deal with. The recreation of reality is much more challenging and that’s why all the tutorials are written for that.
I recommend starting with the tutorial PDF’s in ZBrush 4’s Documentation folder. There are also over 100 free video tutorials available from Pixologic. See my signature.