Hey,
Welcome!
I’m not going to give you step-by-step tutelage but here’s the basic idea that you can follow… read up on specific steps/techniques from other tutorials if you need, and use the adobe Help in AE, it’s really great and should get you going. AE is a big program and you can’t expect to achieve everything straight away.
Because your eye isn’t totally still, you may need to stabilize the footage. Eyes are annoying because you blink, but you will want to track/stabilize based on your pupil, because that is the point which you want to target with the dilation.
To actually achieve the dilate effect, it will depend on what you’ve got in mind but you can probably just use some of the basic distort effects, something like Bulge would be perfect. You essentially want to scale up the pupil area of your eye to make it look dilated.
Good luck!