You'll be ok, don't worry! I know it seems very overwhelming now but like Shaz said, it's a learning curve and then you'll be fine. I was 28 when I had my surgery and had some of the same concerns as you.
I had my surgery laproscopically and was able to get up the next day (with discomfort, of course, but the sooner you can manage to get up the better, as walking after surgery will help you recover faster).
I was able to swim within 3 or 4 weeks. I was mostly concerned about my butt wound from having my rectum removed.
I wear the same pants for the most part. Just had to get rid of a few pairs of shorts that were very low rise. You'll have to see where your stoma is placed in comparison to the waist of your pants. If they can make it slightly below that is ideal. If you can meet with a nurse ahead of time that is a good idea.
I was only 92 pounds the day of my surgery and now it has been a year and I've gained about 40 pounds. Crazy! This is the most I've weighed ever in my life, but I'm 5'6 so I'm still pretty skinny. Plus I was sick for a good long while, so I think this is more like the weight I'm going to be for a while.
Let us know if you have any other questions! You really will be fine!!