When I sing towards the top of my register the notes don't really have much power behind them. They sound breathy and waver a lot. Any tips on how to get more strength in that register? I've been doing a few things like doing onsets, and a few other exercises but I seem to be stuck. Any help would be appreciated. thanks.
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The simple answer would be that you have an issue with your diaphragm support. It's generally easier to have the full chesty tone when you are in your lower register, but it's a little more difficult to transfer that chest resonance up into your head voice.

One exercise that might help is starting in your lower register with a full chesty tone and gradually sliding upward into your upper register. You want to try to keep volume and support consistent throughout this slide. It might help to go slower, you can take a breath if you need to, just make sure you pick up with the same support you had before the breath.
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The difference between falsetto ( that breathy tone) and head voice is cord closure. You need to use your cords to control air flow and not just let lots of air pass through. It's always better if you hear it for yourself so watch this video: