Replying to Cat Experts?

So I just got an orphaned 3-4 week old kitty and it appears that I am in over my head. It wont eat solid food or drink water so im going to the pet store to get a bottle and some cat formula. I've been reading a lot online and it says my kitty could be dehydrated so it might be unsafe to feed him (i think) formula first so it says to feed him a little pedialyte and then formula. How do I know if he is dehydrated? Also any other important things that I might be missing while readiing about raising such a young cat online?