If you're very low level, then you might still be able to contribute with weapon attacks. You're proficient in simple weapons, so pick up a light crossbow. I'm guessing that you probably have okay-ish Dex, so that should help you out when you can't do anything else, at least up until level 3 (at which point your 1d8 damage won't be very helpful).
You get one Hex at first level, and another at second level. Hopefully one of those is Slumber, because you can use that on every single enemy you encounter for the rest of your life, except for constructs and undead and possibly a few other things that are immune to sleep. Try to use it on the enemy standing right next to the fighter, so the fighter can perform a coup de grace on the helpless opponent. Misfortune is another great one, but you really need to take Cackle alongside it in order to prevent the effect from ever ending.
Avoid casting actual spells unless you know that it's going to make a huge difference, or else you will run out and not have the spells when you really need them later on. That's part of the game balance, is that most turns will have a spellcaster doing very little (just defend yourself, use a weak cantrip, or make a weak attack), and then maybe a quarter of them time you get to do something super awesome (like turning a dragon into a sheep, or completely frying a dozen enemy soldiers). If you could just do that all day, with no limit, then there would be no reason for anyone to play a fighter type. The witch actually kind of crosses the line, because you can use a Slumber or Misfortune hex almost every round, and it's a lot more than any other spellcaster can keep up indefinitely.