irishjaneceniza,
No you do not need a brace.
Bracing will only work on children who are still growing, that have not reached skeletal maturity. In young ladies, once they have started their periods (reached menarche), they are considered skeletally mature and will not grow in height significantly anymore.
Also, bracing is usually not considered for treatment of scoliosis until the curve reaches over 30 degrees (or has progressed rapidly in a smaller curve in a young child).
Bracing is also not a cure for a curve. The brace will only correct the curve while it is being worn. The idea for bracing is to try to prevent the curve from progressing any further. When the patient removed the brace, the spine just goes back to the previous curvature. Again, bracing is to try to prevent progression of a curve in a growing adolescent.
At present, you do not need any specific treatment for a curve of 20 degrees. Again, this is just barely the definition of scoliosis. In some institutions, scoliosis is considered any curve over 10 degrees, but in others, a curve of 10-19 degrees is considered spinal asymmetry. Only curves over 20 degree are labeled scoliosis.
The reason for this, is many institutions do not want to give a patient a label of scoliosis, because that might cause problems later on, in trying to get health insurance (pre-existing condition) and getting employment in certain occupations (military, construction, etc). So, you are just barely at the level for this to actually be called scoliosis. Taking measurement error into account, it is very possible that if you have another radiologist measure your curve, it may come out to 18 degrees, or 22 degrees. There is usually about a 5 degree margin of error in measuring spinal curves.
As to what might happen as you get older, it is known that small curves usually do not progress. Occasionally, a small curve will gradually progress over a patient's lifetime, usually at a rate of less than 1 degree per year.
Curvature of the spine, in itself, does not cause back pain or other problems, especially when it is small. Pulmonary or cardiac problems usually do not develop until a curve reaches over 60 degrees.
So, if you are having problems with your back, you need to search for another reason. The most common cause of back pain is musculoskeletal, especially in young adults. If you have done the physical therapy, exercise, maintained your weight, developed your cardiovascular status, and still have back problems, then you might want to see a spine surgeon for an evaluation.
But, as for your curve, it really does not need any specific treatment, other than just watching it over your lifetime.
Good luck.