hi, im new to this board and sort of new to this hobby

So i set my tank up back in april and guided by a friend who had saltwater tanks as a kid we started my 30 galon tank... we got 2 damsels, one domino, and one yellow tail to start out... mid may i added a clown. i have noticed the domino is becoming extremely aggressive to the yellow tail and he can barely eat
so i read that many people take fish back to the store and get new ones.. could i do that with the domino? if so how do i take him to the store? also what other and how many more fish could i get? i have no live rock for my lighting is rather cheap
if i wanted to start live rock and corals where is the best place to find good lighting, what do i need? and how much will it cost about?
sorry for all the questions and the long post
