Welcome to the site. The best thing you can do is read what is on the site. It should give you an idea of what you will need to house and look after your Oscar correctly. As for what type of Oscar to go for, it would just be like if you were buying any other pets, choose one you like the look of. However, make sure the tank is nice and clean and you choose an Oscar that is active and alert. Don't go for one that is just sitting in the corner doing nothing, that's not what Oscars do.
As for filtration, I would strongly advise you to go for some kind of external filter. Choose one that is rated higher than the actual tank. You want to circulate the water quite a few times in an hour so that the water is nice and clean. Like I said, have a read what's on the website and you should get an idea of what sort of setup you need. If you are going to 55 gallon tank, remember that is the absolute minimum for one Oscar so you won't be able to have lots of other fish with it