I actually bought my Mac for Logic a few years back, and I've never regretted it. It's the native DAW, and (not to diss anyone!) but I really think Logic and ProTools are the two biggest, best pieces of software for audio production/recording/sequencing, etc. Cubase, Ableton, Digital Performer and others are great - but anybody in the music industry will know PT and Logic. They are similar in capabilities, but do have their differences - so make sure to google. Personally (surprise, given my nickname!) I recommend Logic.
Next, what kind of music are you looking to do? Will vocals be important to you? Crucial? If you want the cheapest solution, get a USB mic. The AT2020 is a good entry-level USB mic, and you won't have to worry about an interface. If you want higher/long-term quality, you'll need an interface to plug your mic into your computer. And interface has "ins" and "outs." Ins are inputs - mics and guitar/bass, usually, but can be mixers, and outs are usually monitors - but can be other equipment. To keep things simple, a good beginning set up is: Mic --> Interface --> Computer, and Interface --> Monitor speakers. An interface will have numbers for ins and outs in the description. So a 2x2 (2 ins and 2 outs) is a good starting point. If you want to set up a real bedroom studio with lots of mics, you'll want more inputs in your interface.
You'll need XLR cables to connect your mic to the interface. You can get one for $10-20, but don't skimp on quality here.
If you don't already know, monitors are special speakers that give you a clear representation of your music. They start at $150 and get very expensive. Alternatively, you can mix on monitor (not regular!) headphones for $50-$150. This is a good but temporary solution.
Your monitors will connect to the outputs in your interface with either 1/4 inch or XLR cables.
The SM-57 is a classic microphone. Other good entry-level alternatives are the Behringer C-1, MXL V67GS, and the aforementioned AT2020. Do some research there on dynamic vs condenser. Again, if you want to forgo the interface and go with a USB mic, your options are much more limited.
I don't have much to say about interfaces, other than that Apogee is a (rather expensive) go-to for Apple users - you get the quality at a premium cost. There are lots of less expensive alternatives. Google!
You already have a midi keyboard, which is great
For monitors, I suggest you not spend $150 on the AV30s (or something similar) and save up for Yamaha HS50s (or 80s, down the road), KRK Rokit 6s (the most popular option, but I also heard lots of negative things) and Mackie MR5s. If you don't want to jump head first - understandable - look at monitor headphones. I suggest checking out the ATM-30s, 40s, and 50s.
Whew. So, to sum up:
1) Mic (USB or XLR) + cable
2) Interface (optional if mic is not USB)
3) Studio monitor speakers or monitor headphones
That should get you off to a good start. Let me know if I can help with anything!