What causes serve back ache especially the lower back, i cannot stand for sit for a long time?

Many things: Back pain can result from many things such as muscle spasm, narrowing in the spine, nerve root compressions, joint disease, and inflammatory disease. If you can provide further information we may be able to help; otherwise, please see a doctor and have your pain professionally evaluated.

Agree with Dr. S: You can improve your muscle tone and condition by regular exercise. Any type of exercise will do, e.g., walking, jogging, biking etc. It may take a few months of regular exercise to see the full effects. In the interim you may take acetaminophen, no more than 4 grams/day. Have a healthy diet, exercise 30 minutes/day, drink plenty of water, have safe sex, no tobacco alcohol weed or street drugs.