What’s the length? If it’s a short film, ignore all the feature film options.
Are you submitting this film to festivals? You’ll need a short film agreement. Low/No budget.
If you do not plan to submit it to festivals, but do plan to release it digitally, you need a New Media agreement.
If you don’t know, you can file for new media (or short) and transfer the contract later if you decide to go a festival route. If you have time, you can also wait for a new type of contract, which is going to be “agnostic” – you don’t know where you’ll end up.
I’m not sure where you are, or if this is any different outside of LA, but be warned:
SAG requires production insurance and worker’s comp. They also require that your actors be paid as employees, which means not-on-1099s. Which means you have to pay payroll taxes.
Also, if you’re going SAG, you’re going SAG. The other actors will become eligible to join the union.