how to make a trainer?

Start learning the basics of VB.Net/C#/C++ and then move on to a trainer. Otherwise you will just copy & paste sources and you will not be able to fix any upcoming errors.

i just wonder, is it possible to make trainers that unlock stuff and that with cheatengine?

Yes, it is possible.

but it is very hard? or is it the same like i did?

No, it isn't hard.

You just set a 4 bytes addresses value higher. So yea, the same as you did.

lets say, i want to add stuff like unlock all challanges.
how am i gonna do that? is there any tutorials on it? i have searched on google and here. but i can only find how i do the experience and that. if u could help me find any tutorial im very happy

/edit pm or Vm me if u wanna help^^