General

Resist the urge to "reach out" with your head. I assume that's what you're doing when you come back up from a sit-up. Doing a couple with correct form is better than a bunch with incorrect form. You don't have to go all the way down if you're just starting.

Here's a tip, focus on a spot on the ceiling and stay focused on it while bringing yourself up during the crunch. This will keep your head in a stable position which should reduce neck stress caused by accidental neck movement.

You don't need to move your neck while doing sit ups.

Let your abs "sit you up", not your accessory muscles.

Ya i understand that after like 2 sets because I'm tired out I'll start using incorrect form, but any tips on how to focus on using my abdominals?

Like @oAZNvTOFUo said

You're not doing it right if you're using neck muscles. Use your abs. Don't bend your back. Find a way to keep your feet on the ground.

^^ what the above comment said. Just focus on the feeling of "squeezing" your abs.
Try to relax your neck while you're doing crunches by only moving your body. Your head should just follow along.