The Scarlett has a pre amp built into it. Most likely the issue is the distance you have the microphone from your mouth. An SM57 is not ideal for podcast recording because you will need the microphone inches from your mouth, as well as dealing with proximity effect. You would benefit more from a condenser microphone than an external preamp.
You can wire a TRS at one end to link the tip & ring, then to a regular 'guitar jack' TS at the other.
This is basically how a Rickenbacker stereo sums to the mono jack.
Lifted from Cadfael's collection of wiring diagrams (Google translate link to English)