PC recommendations?

I have a really bad pc(Lenovo Thinkcentre) that I want to replace this summer. I want to get a PC that is pretty good on its own(at least decent), not too expensive, and most importantly is easy to upgrade. I am kind of new to upgrading PCs, I have no experience and don't want something really hard. Also, because I can't afford a really good PC, I want something where I can replace everything in it over time and not have to spend a lot at once.