If it does not challenge you, it does not change you.
1. Never miss warm up and cool down: Do not neglect these and do they properly and carefully. You can avoid injuries by warming up and cooling down well. Do dynamic stretching during warm up and static stretching while you are cooling down.
2. What time to workout? Some people believe very firmly that exercise is best done in the morning. It is not false but not 100% true either. Every person is different, has a different body type, different lifestyle, sleep pattern, work schedule. Hence, if for some reason, you cannot get up early and you are not a morning person, it is absolutely OK to workout in the evening or even at night. Idea is to burn some calories, lift some weights and stretch a little.
Also working out during periods has many myths associated with it. While it is true, that if you are very uncomfortable, you should avoid high impact exercises but do not miss exercising completely. Try doing some simple and gentle workout. Working out during periods is beneficial. It warms up the internal muscles and helps to reduce stomach pain and cramps.
3. Doing same workout everyday will not be beneficial- Try to cross train and give body different workouts just as you eat different foods. If you find treadmill boring, try a dance class or yoga class. Variety adds spice and challenge to your workouts.
4. Rest- never under estimate the importance of rest. Not getting enough rest is another cause of injuries. You can also do active resting. It means, resting and not working out the tired and sore muscles and working out some other body part.
5. Staying motivated- everyone needs some motivation factor. Find yours. It might be working out with a partner or a trainer who keeps you on your toes or maintaining a journal to record your progress or a health app that keeps you motivated to go on.