Hi, and welcome
Frosteh has given you some good advice. I'm going to be rehashing some of it here as well as adding my own stuff.
Exercise is good, and will help, but diet is probably 80% of the battle. It's all about the calories. If you're walking for 40 minutes and say (optimistically) burning 200 calories, then drinking four or five servings of pop a day (I have some cans of Pepsi here to hide from my boyfriend who's trying to reduce his intake, and they're 139 calories for 11.6 oz), giving up or cutting back on the pop will do more for you than the exercise, if you see what I mean. (200 burned versus 556 or 695 gained, using this can of Pepsi as the number in question) If you did no exercise and cut it back to one can of Pepsi, you'd be better off. (I'm not recommending doing no exercise)
Having an awareness of your calories (like my little exercise with the Pepsi) is an important part of losing weight, even if you don't want to calorie count (other calorie counters- all have free versions- are fitday, myfitnesspal, and cron-o-meter). You should find a BMR calculator and work out, with activity level (I use one that incorporates the Harris Benedict formula), how many calories you need to maintain your weight and take off between 500 and 1000 from that number. You don't want to go into too deep a deficit or you'll potentially damage your metabolism.
Treadmill/ walking outside is perfect- try to get your heartrate up, so you're starting to become a little short of breath (not to the point where you're causing yourself pain in your joints). Do you have access to weights or anything that you can substitute as weights? (e.g. cans of beans you can hold reasonably comfortably in your hands) If so, then you can do strength training as well, which is important to maintain muscle when in a calorie deficit. There are aerobics routines available online for free (through youtube, or the site bodyrock, and others) for a bit more variety.
With kicking junk habits, try cutting back first (or switching to diet, then cutting back. From what I can see, going cold turkey for most people is a recipe for disaster. So if you have six cans a day (my boyfriend used to do that- I've cut him back from six a day to maybe three a week), cut it back to five, or five and one diet, try that for a week, then cut it back further. (It's slow, but you'll help change your expectations) Also, you may want to find a substitute. While water is the best thing, there's no need to only drink that. Black tea and coffee (by tea I mean hot tea, not ice tea) are virtually calorie free on their own, and adding low fat milk and artificial sweeteners doesn't add very much. Or you could make low cal ice tea. Or there are low cal drink flavourings (squash/ cordial in the UK- apparently something called Crystal Light in the US)
How do you plan to change your diet?
Have a look around the forum for more ideas- especially the stickied posts, the diary section (to see what other people are doing and how it works for you), and the before-after section (for some serious inspiration).
Hope this helps
