Weight Loss Tips????????

I'm 139 pounds and i want to lose about 10 because summer is coming up. i used to be 170 and i managed to lose all that weight but It's harder now for some reason. Do you have any tips on things like what to and NOT to eat, how to stay motivated, and how much exercise i should be doing? thanks in advance (: (also im 14, and 5' 6" in case that helps)