Two things you should know about:
Proximity Effect and Compression
If you already familiar with these subjects, ignore me, but if you aren't, you should look them up elsewhere. They're both essential knowledge for recording (and performing live) vocals.
Proximity effect - when close to the mic (as in 3" or less), you pick up a lot more bass - the closer you are, the more pronounced this is. If you want the full story, read the top post in the microphone forums - its tough to get through, and you can ignore most posts not by Harvery Gerst, but you'll learn a lot.
Compression - a compressor is a hardware or software device that, essentially, lowers the gain (=volume) when the input (your voice) is loud. In your second track, you have sudden bursts of loudness, and compression can control that, and make it musical.
Also, while the SM58 is a classic for live performance, it's generally not used for recording - you'll typically get better sound on vocals with a large diaphram condensor mic - again, the mic forum is where to investigate this. I'm not suggesting you run out and get a new mic - the SM58 certainly isn't bad - just giving you a jumping-off point for learning.
