New player here. I was just wondering, if I wanted to end up as head coach of UCLA for example, and I get offered the 1st assistant job, is it better to hold off for a head coaching job or take the assistant spot (is it possible to get promoted from assistant to head coach)

Taking the assistant's job won't help at all. If you have UCLA listed as your preferred destination job it will be offered to you at some point depending on things like your ratings, experience, success... of course there has to be a vacancy.

I would wait for the head coaching job to become available. I don't think being the 1st asst job gives you any leg up if and when the head coaching job becomes available. The main thing is to be sure your reputation and are high (or better yet very high) for academics in your coaching philosophies so that UCLA will even consider you when the head coaching job opens.