People pick on your weight because it's an easy thing to pick on, and it's guaranteed a reaction. Just because you're not very fit or in shape doesn't necessarily mean you're overweight. And if you arenm't overweight, you don't have to LOSE weight. Make sense?
Basically, they're probably one of -those- people who shoot others down on anything to make themselves feel better. Don't let it get to you.
If you want to lose weight, make sure you do it in a healthy way! Eat more healthily, don't snack, do exercise etc. Like someone said above - walking, jogging and running are some of the best forms of exercise you can do. You don't have to start at an hour a day. Do 10 minutes a day for a week, then 20 minutes a day for the next week. Eventually, you'll be running marathons
Nobody's perfect, and some people just use other peoples' insecurities to upset them. Try and ignore the. I know it's hard, but it's the best way to get through it.
Keep your chin up lovely, and if you ever need to talk, I'm here <3