Press L and you see a blue point on your tow ball, than back up to your trailer and the game will do the rest by attaching it automatically!
Not everything is compatible with everything, there will be a different colored circle if it's incompatible (ex. Purble can't connect to pink, but another purple can)
You have to attach it to a special semi trailer the same way you do with normal trailers and then when you're under the trailer you might need to move a bit front and back so the fifth wheel will connect