As said in previous comments use pcpartpicker.com to help select parts. Kids and certain people like those really over designed cases for the looks, but I say keep it minimalistic for thr case and save about $100 you can then use to make the PC perform better. If you really want to curb how boring the minimalistic look of the case is add some cheap LED strips in good places and the computer will look great if you do it right. If your picking a processor get one of the new AMD processors that just released. If your getting a graphics card get a nvidia 10xx or 20xx series depending on your price range and if he's going to game or not. Dont bother with things like titan cards because they're overpriced for very little increase in performance. Your motherboard doesnt really matter just as long as the stuff your ordering fits. MAKE SURE YOUR MOTHERBOARD AND OTHER PIECES FIT WITH THE CASE. For memory if he's going to game with it then get 1TB or more. For RAM get no less than 16gigs and you (probably)wont need more than 32 gigs max 64. Unless your trying to run real life weather simulations. With RAM keep your sticks consistent. What is meant by that is if your getting multiple sticks of RAM make sure they are equal in performance. For example say you want 16gigs. You can get 1 stick of 16gig or 2 sticks of 8gigs or 4 sticks of 4gigs. don't get something like 2 sticks of 4 and one stick of 8 because then your computer will try and put the load of 8gigs on each of the 4gig sticks. Let's just say that isn't fun. For processor cooling get a paste you feel comfortable with. Some are runny some are thick and wont spread. While installing your processor make sure to wear (plastic) gloves to not get oil on it. If you do get oil on it use rubbing alcohol and a q-tip to clean it off. Make sure your paste is non-conductive so if any spills over it doesnt kill your computer. For the actual cooling unit you can do fans or liquid. You can do basically anything for the liquid in liquid cooling units, hell I've seen people use vodka. Just make sure it's good for transferring heat. (PURE water works wonderfully and its non-conductive) plus with liquid you can do cool twists and turns with the pipes and even line the pipes with LEDs. Also with liquid cooling make sure the radiator can cool the liquid off fast enough to be safe. For air cooling make sure to orient the fan correctly and have a good airflow through your computer. If you ignore this stuff then the computer WILL fry. For power supply basically make sure you have all the necessary plugs and just make sure it can supply ALL of the needed power. With all this in mind go and buy your parts after you've selected them and made sure they fit together. Also please for the love of GOD get warranties they will save so much money in the face of an accident. While assembling the PC keep the boxes incase anything goes wrong. There are plenty of tutorials online if you need help.
TL:DR go back and read all of that because you cant summarize it since its ALL important. If you really dont want to read it all just go and look up some tutorials online but they might not be as in-depth and trust me you WANT in-depth for this.