Hello, there! So I have come to the decision to make my journal a bit cooler and better to look at. So my first try would be at getting more pictures on it, so now I need to know how to take a picture of my self-made application. Here is what i've tried: "Program name"->Services->Grab->...(I can't take screen nor selection) If you guys could help me as fast as possible, I would be grateful! Thanks in advance!

Try Command-Shift-4, then drag a box around your app.
You can also do Command-Shift-4, Space, then click your app to automaticly set the size, but it makes it look a bit odd (rounding of the window corners turns off, I think)

Both methods will put a PDF on your desktop, you can open it in preview and convert to your format of choice.