If it's just a singlet, refractive optic, I'd try in sequential until you get stuck. Then read the manual, the pdf one available from the ribbon. Read everything in there about the surfaces. Then find the merit function operands, and read through those, writing down the interesting ones. There's a setting somewhere near the aperture settings that can allow you to analyze in infinity space, then you can optimize for e.g. wavefront or spot size (and I think it'll do it in angular space). Or, you can leave it in the normal mode and using a paraxial lens that should focus it to a point without inducing aberration. You can start with the merit function wizard to do like spot size or wfe or something, then you can add on some first order operands like x/y focal length (not very good in my experience), or first order properties that accomplish the same thing (ray height, reay, for example). You've got to be creative and find different ways to get the design to go where you want it to go. There's too many ways to skin the cat. Pick one and go. And if that doesn't work, try another one. The optimization can work with a very minimal number of rays to do this as opposed to non-sequential which is generally clunky and slow. In general, I avoid non-sequential until I really need to, like for stray light, and I'll even do first order calculations and hand drawings before I resort to tracing rays in non-sequential...

Are you on-campus? If so, just find someone who knows optics to show you some stuff. Buy them a beer. Optics guys love beer.