Many 'sequels' are simply re-hashes of the first film(s), so you haven't missed anything relevant to them if you didn't see the ones before, such as - OTTOMH - the Die Hard, Transporter, Alien and Hulk films. The type of sequel where you really need to see the preceeding ones are bona fide trilogies where it's a continuing storyline, such as LOTR, Scream, Godfather and - and I'm surprised no-one's mentioned this yet - Star Wars.