Increase Aerobic/Strength Exercise

I want to make a commitment to increase my physical activity to include aerobic and strength training at least 3x a week for 30-45 minutes. 
I know that exercise will help me maintain a healthy body weight, relieve stress, improve mobility, and decrease back pain. 
I struggle with working exercise in my life because I am often mentally and physically exhausted from school, and more movement is the last thing I want to do.