I have just done both of those things this summer.
To stop biting my nails, I went out and got a manicure every 2 weeks, with a bright colour. If I spent $20 bucks on my nails, I wouldnt want to waste it 2 days later by biting them off, and the dark colour looks horrible when you have chipped bits off. For me, biting my nails was a nervous tick... if I was scared, or worried or nervous about some thing I would bite them. So I started sitting on my hands, or chewing gum, or making sure I carried something with me that I could work on with my hands (knitting, or a crossword puzzle, or whatever). If your hands are busy then you are less likely to bring them up to your mouth.
For losing weight, there are really only 2 surefire healthy things you can do. Watch what you eat and exercise. Though you say you dont eat junk food, that isn't the only thing that contributes to it. You might be eating too much at each sitting. It's amazing how much food you can eat without even thinking about it. A good way to work on portions is to take one of those plates for kids that have 3 sections in it... 2 small ones that are 1/4 of the plate each, and 1 large section that takes the remaining 1/2 of the plate. In 1 small section goes your meat, the other small is for rice, potatos (for dinner) or bread. the largest section is for veggies. Lots and lots of veggies. You can eat tons of veggies and not have a ton of calories.
You need to exercise alot. if you can get to a treadmill or can walk for 30 minutes 3 or 4 times a week at a pace where holding a conversation becomes difficult then you will be on your way to losing a pound a week (along with diet).
I decided that I was going to write down all the food I eat in a day and count calories. It was amazing what I was eating in a day. I cut nearly 1000 calories out of my diet each day. that and exorcise has helped me loose 3 pounds a week.
good luck, and feel free to ask me anything!