I'm not too sure what you mean by "loose in the wheels"...do you mean that the wheels feel like they are moving on the axle? If so, this will just need the simple correction of tightening the wheel nuts down a bit so that the wheels still spin freely, but don't make noise on the axle if you shake the skate.
I highly, highly recommend that you not take the stroller out skating on hills until you feel VERY confident on your skates. Have a read over this article by Lorne Milne on Stroller Blading safety
(it's about inline-stroller skating, but a lot of the same stuff applies to roller skates).
If you are wanting to make some adjustments to your skates to slow them down a bit and give yourself more stability, you can try these two things:
1) "Training wheels"
is something we sometimes recommend for new skaters who are feeling nervous. This is when you tighten the wheel nuts down just a bit more so that they still spin, but not quite as freely. Tighten them to the point where they spin perfectly freely, and then tighten them about another 1/4 turn. They should spin a few times, then stop. This will help control your speed while you're gaining your confidence. Be careful not to OVER-tighten them, as you can damage your bearings.
2) Tighten your truck nut
s (the two nuts on the very bottom of the skate) just a little bit more. They should still have a little bit of play in them, but keeping your trucks nice and tight in the beginning will prevent you from being pulled in one direction if you should happen to "flail" when trying to correct your balance. As your confidence increases, you will want to loosen your trucks up to give yourself better turning ability.
I hope this helps! Keep at it, and if you have more questions, feel free to post them!