If its just happened,meaning the pain is acute then best would be to take bed rest for few days as per the orthopedics advice, once the pain has reduced and bedrest period is over ,consult a physiotherapist and take physiotherapy treatment for 10-14 days ,learn the basic back strengthening exercises and continue doing it at home on regular basis.
Next Steps
wear a lumbar support(belt) if sitting/standing for long time also while driving or travelling .
Health Tips
Avoid forward bending and lifting heavy weight,no exercises lifting both the legs together.Avoid sitting without back support.