All of the above advice is good, but put into very simple terms, variety is necessary, using all criteria, length of songs, humour content, choruses, ballads, familiar/fresh etc. Primarily try to choose material you like. Regarding old favourites, by all means avoid the hackneyed stuff, but most audiences like something they can join in with.Try to develop your own style that is different in some way if you want to make a big splash.