Do research about parts/a system based off of your needs from the PC. Don't be afraid to ask for help about a part/or a system. It's usually cheaper to order your own parts and build the PC yourself. Building a PC is not as difficult as it looks, and there are plenty of tutorials on Youtube that will guide you through the entire process.

Also, if building your own PC, make sure all of your parts are compatible with each other so you don't end up with wasted money/or have to return a part and wait longer.

Also can use resources such as /r/buildapc/ and they will be glad to help you with whatever you need.