Ok I'm assuming the little scratches youn are refering to paint swirls which are quaite common on Subaru unfortunately.
The first step is to prevent future swirling. The first step is to look at what towels your using for washing buffing etc. I suggest switching to all microfibre towels for drying,washing, buffing. To apply the waxex I recomend meguiars applicator pads (don't buy cheap autozone ones).
Now to remove the current swirls. Subaru's always have a few and you may never be able to get rid of 100% of them especially if your car is black.
My schedule looks (looked) like this and it kept swirls nearly non-existent. If you can't remove the swirls with hand polish you'll need to try a buffer. Hand Polish has always worked fine for me, so Ill let some one else chime in with buffer info if necessary.
Once a year
1.griots claybar--(removes any bonded surface contaminents form the paint if you need an explanation of how to use it, ask and Ill explain it)
2. griots fine hand polish--(a heavier a abrasive that can be applied by hand, polishes out swirls)
3. Meguiars Scratch X--(another lighter abrasive to finish it up)
4. meguairs NXT--(a regular wax, good for protection)
5. griots carnuba wax--(non abrasive "natural wax" good for a nice finishing coat for that extra shine)
every 6 months
1. Meguiars Scratch X
2. meguairs NXT
3. griots carnuba wax
every 2 months
1. meguiars NXT wax
2. griots carnuba
1. griots carnuba wax
1. wash with meguiars nxt soap
As for the underhood are I'm not a huge expert on this since I've never bought a used car and had to deal with build up dirt. However the main thing I use under the hood is simple green, it cuts dirt very well. I know that alot of detaling places have steam cleaner that will clean it up very well, I know some people have used a power washer etc. but there is a procedure for what to cover etc. Do a search I think its been on here before. Once you do get the build up off a wipe down with simple green once a week should keep things clean.
All griots products can be found at www.griotsgarage.com and all meguiars stuff can be found at www.meguiars.com or sometimes autozone.
Hope this helps if you need any clarification or more help just ask. There ALOT of other producst for wheels,tires,interior,metal etc. I just listed the paint/polishstuff since I wasn't sure if thats all you wanted but if you need a list of that other stuff too just aks and Ill post it.