first of all diets dont work - change your eating habbits and have regular exercise, at least 30 mins per day. the excercise should be enough to increase your heart rate a little. also make sure you drink plenty of water throughout the day.
it is also important to eat enough - your body is a machine and needs fuel. many people think that decreasing the amount of food they eat will loose them weight. well it will - short term though. how much weight do you want to loose ?
here's a little insentive : i lost 5 1/2 stone in under a year - mainly because i was quite big to begin with it is fact that the more you weigh the quicker you loose weight. but in general the quicker it comes off, the quicker it goes on and usually a bit more. i did join a club which was brillient it was rosemary conley. after 3 months i increased my class to 2 a week and so on untill i was doing 5 classes a week. (oh yes, exercise is addictable) it gives you the feel good factor. i learnt about food groups, made tons of friends and became quite insperational to many new joiners.
however, you must take it seriously and stick with it to get results - you will only get out, what your prepared to put into it.
i would suggest that if your very overwieght, unhealthy or have not excersiced - check with your gp that its ok to do so.
good luck and happy weight shiffting.