The 57 is a great microphone for the price and reliability...but I don't think I've heard anyone claim it's the best out there. It'll work great for micing your guitar amp though (it's what I use).
All computer based recording will require a soundcard. There's no way around it. There's no such thing as a "converter" box that would allow you to record on your computer without a soundcard. You might be thinking of a "breakout" box which is a box with (typically) 1/4" plugs that provides a means of plugging instruments into a soundcard...but those aren't available for "regular" cards.
Since you don't know much about computers, you might want to post your computer's specs. Processor, memory, hard drive size/speed, etc, etc. There is a chance that your computer won't work well enough for recording purposes. The reason I say this is that most consumer-level machines have had soundcards as a default option since about 1995....and if your machine is just a few years old then.........
You will need a soundcard but since you're working alone (I'm assuming), it doesn't have to cost much. I get well enough results with a crappy SB16 that was purchased about 5 years ago. The Soundblaster PCI 128 has an ensonique chipset and runs about $30...I've heard good things about this card as a cheap startup.
BUT, you'll also need some kind of powered mixer/preamp to plug that SM57 into. You can't plug a dynamic microphone (or any non-computer microphone) into the microphone jack on the soundcard. UNLESS, you purchase or more expensive soundcard that is built for such a purpose. Now we're talking in the $250+ range.
I don't blame you for feeling like you did your homework. Marketers are pretty slick. "Buy our software and start recording on your PC today!" And then there's the fine print....
I think if you click the "archive" listbox at the bottom of the HomeRecording BBS main page and select "Newbies" there's a long post in there that should shed some light on all of this.
Slackmaster 2000
