Girls easily think that the problem lies with them if a bra does not fit well. But that’s a wrong assumption. Shop around and ask some assistance to find a bra that fits well.

Some common problems:
The back of the bra hikes up. If a bra fits well the strap is straight across the back.
There is breast tissue next to your arm, outside the bra. This probably means the cup is too small or that the bra has not the right shape for you. If it is the right fit there are no rolls of flesh under your arm.
When your breasts are sticking from under your bra the band (and cup) is too wide. If the bra fits well the band under your breast is somewhat tight, but feels perfectly comfortable.