How do I get there if I get off at Powell? Or is there a closer station to Ocean Beach? Thanks 
I’m starting from the Union City station. So I should get off at Civic station, and then go outside to the MUNI? I hardly go to SF, and I usually only go on the BART.
Get off at Civic Center, then transfer to MUNI N-Judah for Ocean Beach from the same station. BART always comes in on the Lower Level of the station. And MUNI uses the Mezzanine Level. The destination signs on the trams will say "Ocean Beach" because it is the last stop.
Edit – Do not go outside at Civic Center. Simply get out of the paid area for BART and find the MUNI gates. Feed $2.00 worth of coins into the slot – there are change machines nearby if you only have bills – and head down to the MUNI platform. The gate will spit out a transfer slip, which you should keep in case you run into San Francisco’s finest. The economy is bad and they have been ordered to vamp up fare evasion enforcement on MUNI. The fine for getting caught without a transfer are $75.