You have several things to decide. Lights, do you what to ge MH, T-5's or CP's. That would depend on weather you want to have a Reef, Fish only or mixed. And if you want a Reef, what do you want to keep. Softies, LPS, SPS. That is why spooda 420 sugest going and picking up the books, So if you know what you want to keep in your tank, you can choose proper lighting, etc.
But lets assume you want to do a Reef, more newbies do. Take your tank, does it have build in overflows. if not, would you want to use a sump, then you will need to get the tank drilled or get over flow boxes and a sump and a return pump. If you do not want to get it drilled then you would need to get a VERY good hang on the back filter (HOB). As far as a skimmer goes, always get the best you can afford. And if your tank is a 58gal then look for something that can handle about 150gals. That way it can handle your sump if you use one. and you will have a nice good quality skimmer for when you want to upgrade a bit (maybe a 75 or 90 gal) Plan on your two biggest expenses while setting up your tank to be your skimmer and your lights.
But also think about a RO/DI filter for your water, or would you plan on hauling water every week from your LFS.
But basically your list will look something like this:
Lights - Metal Halids, T-5's or PC
Filtration- HOB filter or sump,
If you go sump then a return pump
Skimmer - Something at least rated for up to 150 gals.
just my 2 cents worth, stay away from skilters and sea clones,
most people do not have much luck with those. More aggravation
than they are worth.
Power Heads - You will need some for flow. You must have a good
flow in you tank!
RO/DI water filter- just something handy to have, not a must to buy
your own, just a lot easier carrying a jug from the sink instead of
LFS and car, (can't tell you how many times my jug turned over
and I ended up with salt water all over the inside of my car) not
a good thing.
I think that is most of the basics. If I forgot anything someone will chime in. But first get the books, so you can make an enformed chioce. You don't want to spend $300 on lights and find out that those light won't take care of what you want to keep. Then have to go out and spend another $500. That happens all to often, You don't what to spend $$$ to throw it in your closet to collect dust.