The first component (Part or Sub-assembly) inserted into an Assembly is Fixed by default. It will have (f) in front of the file name in the tree. If you drag another component above it in the tree it will still be fixed. If you want to re-position it just right-click on it and choose "Float" from the drop-down. Now you'll be free to left-click and drag it, right-click and rotate it, etc., or position it with Mates.
By the way, if you placed the first component by clicking in the graphics area, which many users do, then it's position will be more or less random. Instead you can select the component to insert and then click on the green checkmark at the top of the Insert Components PropertyManager without clicking in the graphics area. It will then be fixed with it's three primary planes aligned with the Assembly's primary planes. That's what I almost always do with the first component (and sometimes with later components). If I don't do that then I immediately Float it and Mate it where I want. I never just leave a part where it was placed by clicking in the graphics area.