Join a website called SmokinMils, they'll guide you through merching and offer items to flip/merchant, and offer advice on merching.
sometimes its not as fun and you dont get as good of experience in a large clan lik smokinmils, just think of some good items that people use alot, like rocktails, sharks, super sets, saradomin brews, super restores, barrows sets, arrows, runes, and other things of that sort, hope this helped and have fun with your merching!
Like I just said in another thread one of the best things to do is wait until you hear about a runescape update and buy that item to re-sell after the update.