I have 2 single slots (left & right) at bottom of panel but using 12/3 wire for two separate runs
You will need a double pole breaker, or two breakers connected by handle ties for a multi-wire branch circuit.  If you're wiring outdoor receptacles, you'll likely want GFCI breakers, instead of AFCI.  

You should be able to shuffle breakers around in the panel, to accommodate the double pole breaker. 

All that said. It sounds like you are quite inexperienced, and you may want to either do more research, or find somebody who has electrical experience to help you.