Start with lots of searching & reading. There's a tremendous amount of information here on this sub-forum & I found it to be one of the best sources for DIY subs.
I have a similar sized room & decided to go build a vented box with a UM18-22 driver. I used WinISD to model the cabinet & a miniDSP & REW for EQ. I got most of the stuff for the sub from Solen
. The amp, a Crown XLS2002 came from Axe Music via Amazon.ca.
has a huge selection of speaker components, amps, connectors, wire, box flatpacks, etc. Most anything you need to build a speaker. For speaker kits there's Madisound
or DIY Sound Group
& others. I built 4 Atmos speakers with Zaph Audio component kits from Madisound. Very nice speakers.
I prefer to buy from a Canadian source because if there is a warranty issue it's more complicated & expensive to ship stuff back & forth across the border. I did have to return the UM18-22 on warranty. The pricing is quite close once you factor in shipping & customs fees.
I'd have got my Atmos kits from DIYSG, but they don't ship directly to Canada & it is complicated & expensive to deal with that. Madisound & Parts Express make it very easy to get stuff shipped up here.
I've probably given you more questions than answers, but know now that this is gonna be a rabbit hole. Well worth it though.