Building a PC is like Lego for adults. Watch a few tutorials, read the manuals and all should be good. Thankfully the times of molex connectors and CPU heat sinks that kinda needed brute force to get installed are over. Getting a prebuilt PC and upgrading it, is a good start tough.
And like already said, don´t cheap out on the PSU and don´t upgrade before the new stuff releases this year.