1) Read the mission description, it will tell you the sector block and the system you have to patrol.
2)Click M, look on System list; if you don't see the system, then look for the sector
a. if you see neither, open Galactic map and look at the sector blocks until you see yours
b. if your destination is not adjacent to the block you're at, move to the one that is
1. either click the adjacent block, then warp through as prompted
2. move manually in the direction of the next block
3) Upon arrival in the correct sector block, open system list and click the system.
4) If the system doesn't open up when you get there, you may have to change elevation.
If, after following the above steps you can't find the right system, feel free to ask in /zone chat. Be sure to mention the steps you already took.