My advice: Start today! Don't wait until a Monday. When I finally decided to do something about my eating - I realized that starting on a Monday was ridiculous - I started the very next day.
About swimming: I take my DD to swimming lessons 2 times a week at a public pool and let me tell you - there are people of all ages and sizes - no one thinks a thing about it. I don't even really notice. Women of all ages and sizes walk around naked in the showers!

The first time I saw that I was like, "AHH - A LOT of naked people!"

But, anyway, people are typically too caught up in what they are doing to notice you.
Start measuring your food so you know how much you are eating - until you are able to just tell without measuring. I use FitDay to track my food and I LOVE it!
Oh, most importantly, come here - it's a great place to be!
Good Luck!