Car only starts sometimes

My 2003 V6 Mitsubishi VRX Series 2 Diamante Has an issue where sometimes it starts fine and other times it will crank but wont catch, it’ll just sit there making a “nenenene” noise like its waiting to catch. the stereo and windows and lights will still work so i don’t think its the battery. I’m not sure why it does this, and only does it sometimes. I’ve been told it could be the fuel pump, or the alternator or the spark plugs. But i dont know much about cars and would like some advice before i take her into the choppy to get looked at?