What kind of software do you use?
More than I thought use Sibelius, similar to Final apparently (it appears that it's well publicized around students).
The different kind of software you're speaking about after, and the most common, is the "audio workstation", using pianoroll instead of sheet music.
FL-Studio, Reason, Ableton, Cubase, Reaper, etc. for the commercials ones
Macaw or LMMS for the (good) free ones.
They are really similar basically. You can use samples, soundfount, VST (virtual instruments), apply effects, filtrer, mix, arrange, render, etc etc.
The Finale-like (Sibelius, Anvil studio) softs are really different to use than the others.
You can record in it, or use samples from real instruments, or use directly virtual instruments.
Most of them are expansive, or really expansive (a good full-orchestra VST can cost around 800$)
You have some good free ones, but most of them are "electronic" sounding, not made to copy a real instrument.
The hardest sounds to get are the Electric guitar, and the vocals. Even the commercial ones... it's hard to find something good.
Really,trying to have a rock band on your computer is the worst idea about making computer music.
It'll be cheaper for you to buy a fair microphone to record yourself, really. This one for instance is cheap and good : http://www.thomann.de/fr/the_tbone_mb85_beta.htm
You can also directly plug an ampli (or pre-ampli) in your computer using a IN-Line plug from a sound card. Like that, you'll directly record the sound from the ampli (you need to plug your instrument to the ampl obviously).
Cheap soundcards start at 40~80$ (that's really cheap), this one is cheap and good http://www.thomann.de/fr/maudio_fast_track.htm
or this one even cheaper but well http://www.thomann.de/fr/miditech_audioface_ii.htm
"Electric" sounds like 8bit sounding ones etc etc. are easier to get obviously.