Size: Be sure your workout underwear fits well. “For comfort and performance, workout underwear should fit snug, but not so tight they constrict movement or create visible lines on the skin,” Booth ...
In any standing or seated position that you feel comfortable in, maintain good posture (shoulders over hips, spine neutral, ...
All you need is a pair of dumbbells to get stronger in 12 weeks. The 'Woman's Guide To Strength Training: Dumbbells' book will help maximize your gains.