can i tell my laptop which wifi network to use? [View all]

every time i close the thing, it comes up talking to a network that blows. i guess it is my main router, which for some reason is super balky.
running on my timemachine wifi it is fine. go figure.

for about a week after the last system update it held on to the right network, then back to this bs.

is there a way to lock on, or to make this the first choice, or something?