Both you and your opponent each need to have an army built out of suitable models. There is no strict rule what models you should use, but you should endeavour to use models of a similar scale (most commonly between 28 to 32 mm) and have them be armed with appropriate equipment matching their rules, so it is clear to both you and your opponent what each model is supposed to represent. Ideally all miniatures should also be painted, but it up to you and your opponent whether or not this is a rule you want to enforce for yourselves.