Avoiding collisions with SMD components in Altium
Assuming you are using a recent version (looks like V19 you have), use the Tools->IPCFootprint Wizzard in the PCB Library Editor.  That will automatically generate accurate PCB footprints that include 3D models of many parts for you.

Failing that just use the PCB library editor and use Place->3DBody and use an 'Extruded Model' just draw a box the size of the actual part.

Check you have a component clearance rule and that is uses the actual outline of the part rather than a simple rectangluar perimiter.

The right click context menu and "show applicable rules" will help you work out what rule is being violated.

As a takeaway; Altium is complicated, professional software and I really advise you get formal training or at least in-house training if you have a coleague that already has used it. 

As a license holder you will have access to the online training material at Altium Live and that will show you how to do all these things and far more.