I have one front bottom teeth that got loose and fell out. How can I replace it? (Photo)

For your front tooth to get loose and fall out you likely have some sort of periodontal "gum" disease.  You need to schedule an appointment with a dentist and have a comprehensive exam to evaluate this area.  If you are a candidate a dental implant would be the best option.  If not,  you could be looking at a dental bridge or partial denture (flipper). Good luck to you.  

This is a difficult question to answer as we don't really have adequate information.  My concern is why did the tooth fall out?  Often this can be due to periodontal disease.  If this tooth was lost to perio disease other teeth are likely affected.  I would suggest you consult a dentist to see what your options are.  They may include an implant or a bridge but until you know the status of the other teeth you cannot make a good decision.  Good Luck!