Usage of 就 in these two examples
[等 (X) 就 (Y)] = [wait till (X) then (Y)]


Example:

等(你来天津)， 我就(给你当导游。)

wait till (you come to Tianjin,) I'll then (be your tour guide.)

Other combinations:


  [一 (X) 就 (Y)]  = [once/as soon as (X) then (Y)]


Example:

你一(来天津)， 我就(给你当导游)

Once (you come to Tianjin,) I'll then (be your tour guide.)


  [如果(X) 就 (Y)] = [if (X) then (Y)]


Example:

如果(你来天津) 我就(给你当导游)

If (you come to Tianjin,) I'll then (be your tour guide.)