Which should I focus on first: losing weight or quitting smoking? I am 100 lbs overweight and smoke two packs a day and feel overwhelmed by both.

Both. With that much excess in both areas you can focus on both, however you will likely need alot of support. Doctor, therapist, support groups. Perhaps you have an undiagnosed disorder like adhd for example which is causing you to self medicate with food and nicotine. You can do it but dont have to do it alone, and you dont have to be perfect in the process. You just have to remain willing.
Quit smokes please! Smokes for sure need to go first, no questions adked.
Both. If you want to feel better faster, quitting smoking will help that. However, eating well and exercising will encourage good habits, including not using cigarettes. At first you may be very frustrated with your exercise because you will fatigue and stopping smoking will slow weight loss. But stick with it and you will feel much better. See your doctor. They will applaud your decision and help you.