Sodium, in and of itself, will not make you gain weight. But sodium (more commonly known as salt) will make you retain water which will make you look bloated and weight heavier. If you give it sometime, the weight will settle back to normal. But it's better to not have to get to this point anyways. Plus, too much sodium can increase your blood pressure which is something you want to avoid in general. So here are some ways to decrease the amount of sodium/salt in your diet so that you can keep the weight off.
1) Decrease the amount of salt you add to your food during the cooking process. Make sure to measure the amount you're using if you're following a recipe so you don't go overboard. A dash here and there can add up quickly, so be sure to watch it.
2) Instead of using salt, try different herbs, spices, fruits, juices, and vinegars to add taste and flavor to your food.
3) If you gradually start cutting back on the salt, your taste buds will re-adjust so that you won't need to flavor things so intensely with salt. After about 2 weeks, your taste for salty foods will be different, and you won't even miss it.
4) Add fresh or frozen fruits or vegetables to your dishes. These not only have less sodium in them in general, but they tend to be packed with many nutrients your body can use if you're trying to lose weight.
5) Canned foods are notorious for containing a lot of salt. So choose lower sodium versions, or reduced sodium versions. You might even consider rinsing them off before using them to get all the salt off.
6) Pre-packages or processed foods, like the canned food example above, also then to be loaded with extra sodium as a cheap way to flavor food. Beware! Just like above, opt for the decreased sodium version of a product if one is available.
7) Condiments like soy sauce, ketchup, mustard, etc.. also tend to be high in sodium. So use condiments in moderation.
8) When you need to snack, lean on fruits or vegetables instead of those salty snacks.
9) When you're eating out look out for the high sodium foods. A hint might be if they have the following in the title, "smoked, pickled, au jus, soy sauce, in broth, etc.."
There are numerous other ways you can decrease salt from the diet to help you lose weight. The above is a start, but paying attention to the amount of salt in your food from now on will not only help you lose weight, but will also server your long term health for years to come.