You should start by making sure that you know what the definitions of posets and lattices are. Read about them in your textbook or lecture notes, or in Wikipedia. Then you should study several examples of posets that are lattices and are not lattices. See, for example, sections Examples and Counter-examples in the link above. After that, draw the Hasse diagram of the poset in question (draw the elements that are greater according to the order higher and connect x and y such that x < y and there is no z between them). Note that | denotes the "divides" relation. Then, using your experience from studying examples, try to decide whether this is a lattice. Report any intermediate results and questions.