pc build help

motherboard, cpu, ram, power

or if you mean for upgrading then;

power, ram, cpu, motherboard (in case the current motherboard can handle the new cpu and ram)

first must choose psu and motherboard and case

What means what parts to buy before the others? If you are building pc you should gather money and spend them on the build you want, not get 1 part, the other months 2-3... Just gather some money and then buy the parts...

case, psu, mobo, cpu, ram, gpu, hdd, and everything what left with no matter of order.
protip don't take amd sockets mobo.

What is your friend's budget and what does he want to do with the computer?

If you're building a new pc from scratch keep an eye on reddit.com/r/buildapcsales and see if any of the parts you're looking for go on sale.

Otherwise I wouldn't buy a part at a time, you might change your mind later on and regret something, buy it all at once. You could maybe buy RAM in advance, the price is rising on RAM and there isn't much variety there.