If you are under 18, then they aren't done growing yet. Wait until you are fully grown before deciding that you'll be small-breasted for life. Don't try and wear a bra that is too big for you. It will bunch up or it will make your boobs look saggy. Wearing two bras can noticeably make your breasts look instantly bigger. Be careful of this. Wearing 2 bras make you look like you're padded, not natural.
If you wear fitted clothes (skinny jeans, tight shirts) it makes your breasts look bigger. You can also try wearing a push up bra, or a bra that's a little snug than your normal size. Massaging oils on your nipple may also help if your boobs are still growing. Try a few different bra sizes until you find the one thats just right for you. You might also try to have it professionally fit. Once your bra is on, lift each breast into the cup with your hand. Your breasts will appear larger and lifted and the bra will be correctly encasing the breast tissue. Make sure bra straps are tightened well and are not loose. Having tighter bra straps give a great lift. Don't forget that breasts continue to grow until you're in your 20s. So, if you are young, you can estimate how much more you will grow by comparing the size of your breasts to the other women in your family. Also, even though there isn't an ideal breast size, losing weight might make your breasts look more "proportional" to your body. Playing or squeezing your breasts have no effect on breast growth. You can't change genetics by squeezing parts.
To get bigger breasts, eat veggies, fruit, exercise your upper muscles (push-ups), and drink lots of water and whole milk each day. Fattening foods, like extra butter, cheese, and whole milk will put on some extra pounds, increasing breast size in a natural way!
If you have an attractive chest (meaning like no pimples of anything disgusting), always push up on your breasts and sit up straight with your chest out. But not too far out, because people will know what you're doing. Eat lots of fruit and vegetables also drink milk. Remember some of it can make your breast smaller. Stuff your bras with toilet paper or tissues, it is an old trick, and is much cheaper than expensive padding and rubber falsies.. Be wary of stores that carry a narrow range of bra sizes and, instead of telling you your true size, try to get you into a size available at the store even if that doesn't enhance your breast size. If the bra range is slight and you feel uncomfortable, then trust your instincts. Understand that beauty involves a wide array of body types and measurements, and this beauty includes a variety of breast sizes.