I'm looking for software that can record sounds from a MIDI keyboard and perhaps change that sound into a different instrument (like strings or whatever); and also, I would be able to edit something that I've recorded fairly easily.
Garage Band will do this. It is easy to use and comes free with every Mac..
Just the other day I was usimg GB to transpose piano music for woodwinds and some strings. I dupllicated the piano track and then transposed one of the copies then made some more edits.
I'm a beginning keyboard player. My timing is never perfect and I'll sometime play a few wrong notes. So I record myself on the MIDI K/B and play it back and it sounds like a beginning player. But then use the editor built into GB and correct the mistakes and tighten up the timing. Now it sounds like a robot playing the piano with zero expression.
If you ever outgrow GB then buy Apple's Logic Express. It can directly open GB files so you loose nothing. So don't worry about outgrowing GB, there is a painless growth path
About MIDI kayboards, If you need to buy one they start at about $100, some even less. But if you want a "real piano"feel with hammer action keys then the minimum is $400 (casio px120) but maybe double that. But if you are planning to play non-piano sounds (like strings and futes) you may not want the hammer type keys, very soft keys work better.
You just need to get a MIDI keyboard too, not just any electronic keyboard will work. Once you feel you are a master of garageband you can move to Logic if you want to continue composing with MIDI.
This is correct but in addition you can record real instruments. Like a real piano, voice, a flute or drums. For these you would need a microphone. Garage Band treats these kinds of tracks a lot like NMIDI and you can edit them. Cut and paste and even adjust the key or pitch within limits. So technically you don't need MIDI. You can get starting gust by singing into the Macbook Pro's built-in mic. That wil not get you pro-level sound quality but you can get started by simply setting the notebook on top of the piano and recording with the built-in mic. Yes I know, very poor audio quality but it's something to try with a brand new out of the box Mac