If you've joined KiwiSaver for the first time through a provider, your employer will be contacted by the Inland Revenue with instructions. To avoid delays, it's best to also notify your employer directly as well.
If you've started a new job, you're employer should give you a KS2 form to fill out. This will tell your employer your contributions rate.
If you want to change your contributions rate, you'll need to fill out a KS2 form and give this to your employer.