I learnt something and I only know the beginning few steps
1. Load your video
2.Create a new layer - solid- choose a colour
3. Create a mask
Now I'm supposed to create a sqaure mask and it should be for example a red square on my video that I have to track but it's just a red layer with the shape, it wont crop the square so i can see the video.
I got it to work once but I dont know what I did.
Sounds like you want to use the solid layer as a track matte. Press F4 until you see track matte options at the bottom of the timeline. Select the solid layer as an alpha track matte for the video below.