Basically ALL parts are created in separate files and these parts are inserted into assemblies (it's the same as xrefs). All parts need to be fully constrained in 3 degrees of freedom, if you don't it will 'fall over' as they say. You can bend the rules but you need to be pretty competent and understand the side effects.
- have a good system for naming and organising part files, maybe include type codes and other to quickly identify parts in your part tree and job file/s
- set out your parts using logical work plane locations and orientation, you will soon find this out when adding parts to an assembly. It also aids in setting constraints quickly. A little bit of planning goes a long way.
- learn about visibility and other 'states', it's like turning layers on and off.
- like visibility states etc you need to organise your assembly part tree, you can move the'end' marker up and down to remove/add parts for ease of editing.
There's a lot to learn but the first 2 tips above save a lot of headache, just remember that all parts are 'referenced' in and that if you edit one 'instance' of a part it will affect all other instances and could have undesirable side effects.
good luck.