To read Sinhala content you shall install Sinhalese fonts, please follow steps given in
සිංහලෙන්ලිවීමසඳහාපලමුව ඔබේපරිගණකයේ
Once it is done you can use online tool http://www.ucsc.cmb.ac.lk/ltrl/services/feconverter/t2.html
Where you would type phonetic Sinhala , means
For example...