I second using a site like pcpartspicker. It can help you avoid some petty technical mistakes, like buying an under capacity CPU cooler, or a power supply without enough of the correct connectors and voltages for your cards.
Sadly, it doesn't actually help you avoid the most annoying mistake — when things just don't fit into the case. They were basically completely wrong about fitment on my PC parts build. For example, the cooler they said would fit wouldn't fit, and the cooler they said wouldn't fit in fact would. They also in many cases have incorrect fan mounting information; they got the size of the front fan mounts on my case completely wrong. If I had gone with liquid cooling, I would have ordered the wrong cooler on their advice as a result. As it is, if I hadn't watched Youtube videos, I wouldn't have known that they got the air cooler wrong. In addition, they claimed to have lower prices for all of my hardware than what I could find myself, but literally all of their advertised prices were false. On some six pieces of hardware, none of them clicked through to the listed price.
This isn't to say not to use them, but you will want to check up on literally every piece of information they give you, because it may well be wrong.