Check out love2pedal.com ... for shorts. Lots of people here have had good experiences, myself included.
re: your seat, seats with lots of padding often result in pain oddly enough ... a pair of bike shorts will help some in that regard, but seat time will also. You'll develop "butt calouses" in time.
What size frame is that? Is your seat too low? When you're at the low end of your pedal stroke, with the crank arm in line with the seat tube and the ball of your foot on the pedal spindle, your leg should have a slight bend to it.
Re: other cyclists, ride solo if you prefer. Many of us do. You can also look to things like meetup.com to find organized rides that cater to many different fitness levels. Don't jump into an organized ride (especially "fast" rides) until you think you're ready, and even then, you might want to wait. It can be demoralizing to join a group ride and get dropped from the outset.
Drills to do when off the bike? Put the fork down and push the plate away. Track your calories in. You can't outride a bad diet (and I personally have a VERY hard time with this drill!).
General advice? Listen to your body. If you're in pain, take a break. You can "override" in your eagerness, but remember, our bodies need time to heal as well as time to work. Learn to spin rather than mash (some will disagree with this -- but it works). Wear a helmet. Don't give up when you get discouraged. Ride.