You can learn to assemble a PC on your own.
When building a rig, two things are absolutely important: the first is "budget"-- how much are you willing to spend; and the second is "purpose" -- what do you want and need to do on the computer that you're building.
As for choosing parts, it also boils down to those same two things. In other words,
1. What do you want/need to do with the computer?
2. How much can you spend for it?
You can either ask a friend to show you, or watch video tutorials online. Some motherboard manuals have instructions on installing components like the processor, heatsink, etc.
Make sure that the parts are compatible with each other: processor and motherboard must be of the same socket, RAM type must be correct, etc. Keep everything in their boxes and anti-static bags until you're ready to build.
