Over weight and need to lose 25-30 lbs

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Hello everybody! Right now I'm 173lbs 5"6. 2 years ago this fall I weighed about 130lbs and then I gained 20 lbs over the winter and then gained another 15 lbs or so so now I guess I'm over weight for my age, (under 18) and now I'm trying hard to get to about 140lbs so I can be healthy and not have to worry about heart problems as I get older! So any tips or words of encouragement would be much appreciated! :D

Take care