How do I pick the right ATC channel to listen to my flight?
As you would have noticed, it's not published which frequencies your flight will use. So it requires dedicating some time on your part (to some, it's time well spent).
The terms

Ramp control: For the big airports, those frequencies are typically not staffed by FAA controllers, but airport/airline employees handling a section of the airport ground where the planes park and push back.$^1$ Also see: How do terms apron, ramp, tarmac, taxiway, parking area, gate, hardstand relate to each other?

Ground: For the big airports, there may be more than one, say one for the northern part of the airport, and one for the southern. Figuring out which runways are used when (time of day; winds) will make it a bit easier. Ground handles ground movement, anything that is not ramp or runways.

Tower: Same as ground, there may be more than one, and this one is for runways (taking off and landing).

Departure: This varies wildly, even for a single airport, it will change depending on the winds, time of day, and traffic load. It may even be combined with arrival/approach. This is the airspace around the airport outside the tower's jurisdiction, which connects to the en-route/center controllers.


Figuring it out

Find out from which gate/terminal you'll board.

Start listening to all channels days ahead and try to find your flight. You can search for archives of recordings too (there is no need to free up your schedule).

Take notes of the other flights as well, and where they park. Some flight tracking websites show the planes that are parked on the ground (playback/archive functions exist on some as well).

Use the 4-letter ICAO airport code, in this case KSFO, to search for free parking charts. Plenty of flight simulation groups release custom made versions of those charts, but of course they are of no value for real flying. Note: FAA charts are public domain, but as far as I know the ones with the parking stands are not published.

Get the rest of the charts, and combined with the above, start learning how the airport operates.


Easy way 1
If your flight will receive its flight clearance by voice radio (computerized/data clearances are taking over) on the delivery frequency, just listen closely and start switching frequencies to match those given to the pilots (e.g., United xxxx, contact tower on xxx.xxx). If not, once you catch your flight on a frequency, start frequency-hopping with them.
Easy way 2
If United still offers &quot;Channel 9&quot;, just cheat: How was I able to just plug in my headphones and listen to ATC and pilot chatter?

$^1$ https://www.nap.edu/read/24668/chapter/15