Hey Jacquie! Im 19 and I can remember all too well what it was like for me at 14. I have always struggled with my weight a bit, all my friends are naturally petite and I had a few extra curves and that made me self conscious, especially at the pool!!!
At your age (not to sound patronising at all!) I would suggest just watching what you eat, and not a strict diet, as your body is still growing and it needs all the good things food can give our body, so its not a good idea to start cutting out whole groups of food.
Having said that, just try to make a conscious decision to eat healthier. You can still have your soft drinks and chips, but save them for the movies and girl nights, so they are treats. During the week, limit yourself to one biscuit instead of a few, drink PLENTY of water instead of soft drinks or juice which contain lots of sugar and extra calories and have healthy sandwiches and fruit at skl instead of buying your lunch from the canteen. Simple things like this will really help! Plus exercise and you will have that flat tummy in no time!
Good on you for going to the gym! Thats great! Exercise does make you more hungry, however just be aware of when you are actually hungry and when you just feel like eating because you can.
When I was 14 I was desperate to lose weight for the yr 10 formal, and I did! Just by making small changes and going to the gym for an hour a night, 5 days a week. Also, watch your serving sizes, and maybe serve yourself slightly less so your not tempted to eat it just because its on your plate, and not because you are actually hungry!
Good luck! And remember, confidence is the key! Guys know when a girl is confident and they are attracted to that. I know from experience, being shy and self conscious myself. I didnt start dating til I was 17, but now I have a fab bf and I am so happy. Besides, my bf likes the fact that I havent been with heaps of other guys, its a turn off! So dont worry about it! Wait til you find someone who is worth dating, and not just the first person to ask you! You'll thank yourself later!