I've bought a scarlett 2i2 and SM58 today, but it seems the volume recorded is quite low when compared with the CDs produced by industry. Do I need a preamp to increase the volume?
The short answer is, "no." Low levels are normal at tracking stage. If you are running somewhere around -18dbFS that is about right. You're dealing with the first of four stages: tracking, mixing, tweaking & mastering.
When you get to stage three it SHOULD still be lower than commercial CD because the job of the mastering process (among other things) is to raise the volume (via limiters/compressors and other dark magicks) to that commercial level.
John is correct on the preamps. You have two. If you wanted to use an outboard preamp, you would plug it into the line input of your Scarlet, not the mike input.
You shouldn't need to if you are running around -18 to -20dbFS. Everything will come up through the stages.
Know the four stages and use your volume control on your interface at set levels for each stage. I have put white marks on my volume knob for the first three stages. First is high (for tracking) Second is fairly low (for mixing...note that it is extremely important to mix at the same levels most of the time as your ear perceives things differently at different volumes) Third stage is back up a little, but not like first. Mastering stage (if you choose to do this yourself) should be at a level that is comfortable for the song, easy to listen to and gives you a good feel of the overall mix. Mastering is kind of genre specific. There are certain expectations for different genres. You would not master a jazz mix the same as a death metal mix. You would however, track them the same (maybe not the same timbre, but you get what I mean. You put a mic in front of an amp or guitar or voice and get the best sound/performance possible. That's tracking
On the shelf means physically on the shelf. Available to be purchased from the shelf of your local retailer...Love the English language. Shelving is something to store books/products on, but also something you do with equalizers...
Don't know what DAW that picture comes from, but that is a GREAT looking signal. No clipping, everything right with the world. You've got a good noise floor and adequate headroom.
-20 is less, but your DAW is not showing dB meter, showing digital (0-1).
You could post an audio sample for us to hear, but this is not hissyfit stuff, meaning, if it ain't hissy, don't have a fit.
You've got a good signal. Take that into a mix. Push the mix around, EQ and effect where necessary. Then hit the MP3 clinic and let's see what it's like!