Im 18 years old, and trying to build credit to move and find an apartment in another state. I need to be ready to move by june to attend college in the fall, Im thinking of asking my older cousin to co-sign for me. So my question is how long do I have to wait to build credit after he co-signs for me? or do I get credit soon after he co-signs? Im new to this so I dont know how things work after he co-signs the apartment for me. So please help me out! Thanks!

Cosigning for an apartment won't affect your credit. I have never heard of an apartment that reports to a credit agency.