You can do it without joining, but you’ll need to do some work first!
Replacing some meals will help, but to see results you’ll need to follow the plan. It’s very easy, and I’ve always had great results on it. Do a wikipedia search for weight watchers. On there they have the formula for working out WW point in food.

Basically, you’ll need to go through everything you eat, noting down the Saturated fat, total calories and fiber per portion. Then use the formula to work out the points per portion. Write this down.
Eat 26 points per day. You must count everything that you swallow, including cod liver oil capsules if you take them, and drinks. Every 15lbs you lose, take off a point per day. You can save up to 4 points per day, and eat them another day. You must eat all your points by the end of a 7 day period.

Try to exercise as well, this really helps.

Good luck!