how to model in inventor

I'm always facing a BIG problem in inventor (I'm a beginner tho). I create an assembly, make a part in it, then I create a second part, but when I edit the second part, I want to use the first part as a guide, BUT IT'S ALL in Ghost mode and I can select any of it !
Please help !
4 Answer

thanks for answering william,
What i mean by "ghost mode" is transparent display of the part. And the impossibility to select edges/points/faces of that part, to use them as guide, to constraint the sketch of a new part...
in an assembly, I create a part.
then i create another part, so I'm now in sketch mode
I would like to create a sketch based on the first part i made (logical)
I can't because when I'm editing the sketch of the second part, the first part is not selectable.
I tried looking into forums for my answer, but I couldn't find any solution.
I'm pretty sure it's a basic thing to do. I use to work like that in solidworks.
looking forward to reading any advise on that matter, thanks.