Opening a sliding window from the exterior is something that you hear a lot about when it comes to burglars and special forces operations, but if you step back and look at it then you realize it's much harder than you think. With a few simple tools you can sometimes open a window lock, and then push the window open, but it's not a simple thing to do if you're not familiar with the window.
Check to see if the window is open. If the window is unlatched then you might be able to slide it using the friction from your hands. If not then you could slide the prybar between the windowsill and the window itself and jiggle it till it moves. If the window is latched, this approach won't work.
Work a thin hacksaw blade into the window near the latch. This only works in situations where you can flip the latch by moving the blade straight in between the sash and the window. If you can reach the latch, push it with the hacksaw blade till it's open.
Break the lock. This isn't a good option if you want the window to still be functional once it's open, but if you work in your prybar near the latch and push hard you can likely pop the lock open. There may be additional locking mechanisms you can't reach, such as dowel rods, that will still stop the window from opening however.