I would start with about 7’ triangle, say three feet forward of wall behind speakers. Set up speakers to start with the beams crossing about a foot behind your head. Just a starting point.
Position your triangle. so you can treat the first side wall reflections and wall behind speakers. So any door should be behind the listener somewhere. Twelve feet is small enough you will probably want to treat the wall behind your head with absorption. If floors are not carpeted then you want a thick area rug. If there are windows behind the speakers, you’ll want to either put in heavy treated curtains to absorb... or just partially cover up with acoustic columns.
Do not put anything except the amp in between the speakers. Put equipment racks off to the side... and away from the front corners.
I have helped a friend of mine assemble and then treat his similar sized room. It was actually rectangular, so for a month it would be one way, then a month the other, until the final configuration. It took over a year... don't be in a hurry. It will be a learning process. Don't buy acoustic treatments at first. Use couch cushions, blankets, and pillows you have laying around the house to learn the room and to get the configuration right. Then you can buy a couple acoustic cylinders or panels at a time to get it right.
Take your time. Consider this your new hobby... what it will sound like after versus before will be completely unbelievable.