Heat beds typically aren't really polarized, so any + and - reference doesn't matter. The only situation I am aware of where it does is if the heat bed is equipped with an LED to indicate when power is on.

With RAMPS, people normally wire the SSR control inputs to the D8 output that would otherwise power the heat bed. For the D8 output to work, you need 12V connected to the 11-amp power input.

I think most people get an SSR off Amazon. You can search the Makerfarm subforum for "solid state relay" for some threads that provide links. The general comment I'd have is that going higher in a current rating is better, since the higher current relays will typically also have a lower on-resistance. This maximizes the amount of power going to your heat bed and minimizes the power lost as heat in the SSR. You also want to make sure the SSR is for DC loads, not AC. Unlike mechanical relays, that makes a difference for SSRs.