Those black speckles in the photo on the lower edge if the wheelarch are corrosion in the metal. I have seen body places rub down surface rust (the actual brown rust that we all know as corrosion) but then paint over the remaining black speckles as the surface is smooth after sanding. The owner gets his car back all nice and shiny, only to find within a few weeks bubbles appear again. These black speckles in the metal have caused this. They need to be ground out if just surface rust, cut out if from underneath or at the very least treated, or any paint applied, no matter how good, will bubble in a short space of time. The black speckles can be very very miniscule, but need to be fully removed by whichever method is appropriate before painting.
Being honest Lizzie, from your questions, I would leave it as if is for the time being or you will end up with a wheelarch looking far worse than it already is, and whenever you are able, get the job done professionally, enduring that a proper job is done, rather than cosmetic.
Unless you have a good deal of experience in bodywork, whether that be DIY or professionally, especially with metallics which are two coat systems (base / colour coat and then laquer, and that's after primers etc) the result will not be what you expect, and you will regret attempting it! It is an art getting a perfect unblemished finish that comes with plenty of practise and experience.
When you are out and about, have a look at older cars say in a supermarket car park - there are certain to be some where the owner has attempted a repair, and see what you think of the result.
Alternatively if you can get your hands on an old door panel for example, put a couple of dents in it, and practise repairing and painting that first before your own car! Then make your decision about your abilities before starting your own!