Quickest way to get an online store going

I’m part of various online communities that I would estimate today at about 50k very motivated / dedicated people. These communities are very focused on specific types of activities. I believe they would represent an excellent customer base for the typical things like T-shirts, mugs, … etc. I’m looking to start an online store and print motivational stuff / gear for this community and find ways to advertise it on their website/community boards/…etc.
I could research and do this myself, but I have too many other obligations at this current time. I really just need someone to lead me in the right direction so that I can get this off the ground and running within 10 days (at most).

I’ve used T-Shirt gang in the past, but also others exist with similar products.