Depending on how much you weigh, experts recommend you aim for losing about 10% of your body weight over six months time. That way you don't resort to unhealthy dieting and miss out on valuable nutrients; you don't lose a lot of water weight that tricks you into thinking you are doing things the right way; and you make small changes and lose weight slowly. So the weight stays off and the healthy new habits stay with you. This means no more dieting in the future!
If you weigh 250 pounds you should aim for 25 pounds off in six months. This is just about one pound a month which should be quite easy to do by eating a little bit less (either smaller portions, lower calorie foods, healthier snacks, or some other ways of cutting back what you take in) and exercising a bit more (just going for a 20 minute walk, parking farther away when you drive somewhere, taking the stairs instead of the elevator). These are all habits you can start working into your daily lifestyle that won't stop after you've lost what you want to.
It's tempting to want to get off five or six pounds a week, but this can't physically be done unless someone is extremely overweight, severely limiting nutritional intake or exercising for hours a day--actions that can't very well be expected to become habits. This leaves us right back where we started as soon as we relax our rigid expectations. And most of the weight loss we see on the scale reflects water loss--not what we really wanted to lose in the first place!
I would suggest you sign up for "5 tips to set you up for successful weight loss" at my website (http://www.mycoachlaurie.com) to get yourself in a great and ready frame of mind. As you go along, view the blog "Balanced Diet, Balanced Life" at http://lifedietbalance.blogspot.com to fill you with ideas and motivation over the next several months.
Good Luck! And do let me know how you are doing :)