Meeting any celebrity is very difficult unless you just happen to be at the right place at the right time. One way to do this is to attend concerts, but that still doesn’t ensure that you will meet the pop star. Sometimes, you can attend multiple concerts and over time you find the right areas of the concert arena where the celebrity enters and exits.
Then you could meet them then. You could also try to get a back stage pass which is difficult to get. There are ways to meet people like Bruno Mars without attending a concert. If you live in the same city that he lives in, you can try to frequent the restaurants and stores that celebrities frequent and hope to run into him.