I'm new to the forum but not to fitness and nutrition.
Swimming can help with weight loss and provides an alternative to other cardiovascular exercises such as running, aerobics, and biking.
However, swimming is a skill that needs to be practiced and honed each and every time you get in the pool. Most of us know how to run, jump, twist, turn, and bike. But, the skill of swimming is not something every one has. But, is something that everyone should learn.
You will find that by adding swimming to your exercise routine that you will burn additional calories. As with any exercise you should try to spend at least 20 minutes actively swimming. I would shoot for more time than that 30min +.
However, there is something that you need to very well aware of. You cannot out run a fork. Weight loss is a very personal thing that means different things to different people. If you want to lose weight that is going to stay off you really need to look at your diet. Weight loss, for most, is a simple equation. Calories in < Calories out. The best way to figure out exactly what you are eating is to keep and honest journal. There are many ways to do this. There are even great phone apps that can keep up with it for you. I personally like myfitnesspal. I am sure others will chime in with what they do as well.
I am sure there are plenty of folks on here who know a thing or to about swimming for health and weight loss. Post up where you are currently and what your goals are. That should give us something to help get you started.