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, and it will remain fixed regardless of its position 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, hold down the scroll wheel to rotate it, etc., or position it with Mates. (Placing your cursor on a component, holding down the scroll wheel, and moving the mouse to rotate the component is the default behavior for a standard 3-button or 5-button mouse. If you have a more elaborate mouse, or have changed the button functions, the behavior may be different.)
By the way, if you placed the first component by clicking in the graphics area, which many users do, then it's position may be more or less random. Instead you can select the component to insert and then click on the green check mark at the top of the Insert Components Property Manager 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.