To improve your cardiovascular fitness: A. Include a minimum of 20-30 minutes of aerobic activity 3-4 times a week.
B. Use weights or resistance to build your muscles.
C. Perform push ups and curl ups daily.
D. Stretch every day.

A. Include a minimum od 20-30 minutes of aerobic activity 3-4 times a week

Explanation:

Though all answers will help your body and cadiovascular system, A seems like the best answer since it will help improve your system the most.

i can't answer your question in very sad situation.

but if your situation is very bad, then you can find happiness by thinking about your past that has full of happiness.

or you should do meditation for some time to get rid off from that bad situation.