That doesn't strike me as unusual. The SM58 is a dynamic microphone (i.e., “inverted loudspeaker” design), and the main reason it's so popular is that it can take quite a beating (both in terms of sound-pressure and mechanical abuse) without problems. IOW, it's not very sensitive. That doesn't mean it can't also be used for harmless sources like normal speech in a studio setting (in fact that's what it was originally designed for!), but it does require a lot of gain then. A good mic preamp should be able to offer that, but a built-in one on a cheaper audio interface may introduce a significant amount of noise if you try it.
In studio situations, for sources other than drums or electric guitar, condenser microphones are usually preferred. These have a much lighter diaphragm, hence are more fragile but also capable of picking up fine sounds more accurately. They need a phantom power supply, both for polarizing the capsule and for the built-in stabilisation stage; most audio interfaces offer that.
So, if you're serious about this you should probably invest in either a cheap condenser mic, or a decent mic preamp. (In the long term, in both.) For vocals, a tube preamp tends to make a lot of sense (it adds a bit of saturation harmonics when driven, which further adds power to the signal psychoacoustics-wise).
In the meantime, make the best you can out of that signal. For vocals, a decent compressor works wonders for bringing up the signal level. Audacity has one built-in, but its quality is unfortunately horrible. I recommend you check out Reaper, an excellent DAW that comes with good plugins for compression, denoising etc.. It's not technically speaking free software, but you can use the trial version just like that without problems.
