A lunch you would recommend for somebody trying to loose weight?

I usually have a ham sandwich on multigrain bread with yoghurt. but it doesn't seem to be doing much just keeping my weight the same. I am active, I go for morning runs/walks for a couple of kms. I read somewhere that you should be having smallish lunches perhaps mine is too big? I am only 18 and female. And pretty small for my age. What would you reccommend? I have a whole bunch of cut up fruit in my fridge (watermelon, kiwi, pineapple etc.) but I've read multiple places that you should also be watching your fruit consumption, it is so confusing. I also have diet yoghurts at 50 cal per serve, but I'm sure next I'll be told these a toxic for my health also. What can you reccommend me? and how many calories should I be consuming at lunch time if I try to loose weight?