As to the losing weight... it depends. And it depends on so many variables that I couldn't say.
However, it's February now, so August is about 6 months... to lose 15 in that time, you'd need to lose about 2.5/month. That's a pretty high target for any sustainable weight loss plan (note the word "sustainable," which leaves out crash diets). For most of us, the weight didn't go on fast, and it won't come off fast.
But by August you could
certainly be smaller-than-you-are-now, and on the way to better habits and a long-term healthier life.
Adding exercise is a good idea. Exercise can make you look a good deal slimmer, even if the scales don't move.
You might check out the "Shovelglove" forum. A lot of folks on this board walk, some with pedometers. Others do "14 minutes of anything" - Shovelglove, yoga, Pilates, whatever. Just remember "something is better than nothing" and "daily babysteps beat one-day sprints".
Hope this is helpful for you. Welcome and good luck!