Learn Dutch: Ik zal je morgen steunen. - I will support you tomorrow.

Because we are using the auxiliary verb zullen (here conjugated as zal) to form the future tense. In Dutch, when you use a helping verb like zal, the main verb (steunen) is pushed to the very end of the sentence in its full dictionary form (the infinitive).

Could I use gaan instead of zal here to talk about the future?

Yes, you could say Ik ga je morgen steunen (I am going to support you). However, using zullen often makes the sentence sound more like a firm promise or an assurance, which fits perfectly when offering someone your support.

Why does this sentence use je instead of jou?

je is the unstressed version of the object pronoun jou. Unless you want to put special emphasis on the person you are supporting (as in "I will support YOU, but not someone else"), it is much more natural and common to use the unstressed je.

Why does morgen come before steunen?