How much do you game? How much do you type? Have you used a mechanical keyboard before, and do you have any preference on switch type? Does it need to be backlit? Should it have macro keys? Do you want media controls? Do you LAN party (so should it be more portable)? Do you want the full keyboard with number pad and everything? I'd check our /r/mechanicalkeyboards for help on this one.
What grip are you (claw, palm, fingertip)? What kinds of games do you play (fps, rig, mmo, moba)? Do you have a colour scheme that you're going for? How many buttons do you want? How big are your hands? Do you prefer lighter or heavier mice? What kind of sensitivity do you need? What surface will you be mousing on (these last two should help you decide laser vs optical). I'd check out logical increments's mouse guide for everything you need to know.
Peripherals are all about preference. The best way to know what you like is to try it out. I recommend finding a store nearby where they sell gaming gear, and try different stuff.