Part 1:Spotting Signs of Breast Development
Look for breast buds poking through your shirt. If you see these small breast buds, it’s time to get a first bra. Breast buds are small bumps that appear under each nipple. However, if a girl is feeling self conscious about her breasts, it might be time to get a bra - regardless of physical development.
You should feel some tenderness or soreness when breasts begin to bud. This is normal, and nothing to be concerned about. It just means you are starting to develop.
Next, the nipples and areola (circle of skin around them) will get darker and bigger. Then the breasts will start to grow more, at first perhaps having a pointy shape.
Understand the average age that girls develop.
The average age for a girl to start wearing a bra is age 11. Some girls need one by age 8, though, and some girls don’t need one until they are 14. Every girl is different!
Sometimes girls who aren’t really that developed ask to wear a bra because their friend is wearing one. They could start out by trying a training bra
You could also start out by wearing a tank top under your shirts. Either way, you should not stress out about whether you are developing like other girls. Everyone moves at their own pace, and that's OK.
Recognize the signs of puberty. Developing breast buds is just one of the many changes a girl will see in her body as she starts to enter puberty.
She might start to grow pubic hair. In some girls, pubic hair actually develops before breast buds appear.
Puberty can cause a girl to gain some weight, especially in the belly area. The belly might look more rounded. This is a natural sign that a girl is starting to mature physically.
The girl also might start menstruating, although her periods might be very infrequent at first. All of these things are normal signs of puberty.
1 Try a training bra for your very first bra. Girls can wear a training bra/crop top type bra when the nipples start to protrude. These bras are a lot more comfortable, and they almost look like crop tops, so you might feel less embarrassed because they won't be as obvious.
You want to find a very comfortable bra for your first one. There’s no reason for a young girl to have a fancy or lacy bra. Training bras are usually simple, stretchy cotton bras without cups.
A sports bra is another good idea for gym class or if you are in any team sport. Because sports bras are designed to be flatter in the cup area, and are very comfortable, they can be good choices for your first bra, even if you don’t play sports.
2. Choose a bra with a soft cup if you've developed further.
If you have grown breast tissue beyond the buds, and if you measure an A cup or more, it’s time for a soft cup bra.
Measure yourself or ask your mother to measure you every four weeks so you can figure out when you need a soft cup bra. These don’t push up or otherwise change the shape of the breasts, so they are appropriate for young girls and are also comfortable.
Underwire bras are also not a wise choice for a first bra; they provide more support for girls with very large breasts, and since you're just starting to develop, that's probably not needed.
You can choose a bra that’s similar to your skin tone so that it doesn’t show through your shirt, if you want. Buying bras in different colors can help you match them to the shirt, so they don't show through (for example, you wouldn't want to wear a black bra with a white shirt unless you have darker skin).
For example, a girl might need to know that you don't need to wear a bra at night. Some bras have padding, but others don't, and the padding is not necessary when a girl is just starting to develop.
You can use a wash bag to protect the bra from damage during a washing cycle.
You can find first bra lines in many department stores and in many lingerie lines. This can be a good choice for a girl's first bra.
Learning More About Bra Size
Ask your mom or another adult about puberty. For many girls, getting her first bra is a complicated experience. You might worry that you will be teased by boys or other girls if you are developing faster – or slower. Understand that these sensitive feelings are normal. Who knows? Your mother might bring it up to you first.
Ask your mother or another adult for a book that explains puberty. Explain to her what is going on with your body.Be open about your feelings. Sometimes boys tease girls about bras. If this has happened to you, don't worry - it's common. But do tell an adult.
Realize that women are beautiful no matter the size of their breasts. Girls might be worried if their breasts are small or teased if they are big. Realize that women come in all shapes and sizes.
Don't worry if you feel embarrassed. Understand if you feel embarrassed that this is normal at your age.
If you are the parent of a girl, don’t discuss the topic with others – friends, siblings – in front of her.
Understand how bra size is determined. You want to make sure that you pick the right bra size so that you are comfortable and properly supported.
In the US, bra size is broken down into two pieces: Chest size and cup size. The chest size is an even number, such as 32, 34, 36 and so on. The cup sizes are in letters, like A, B, or C. In other countries, like the UK cup sizes can be different (AA, A, B, C, D, DD, etc.)
A saleswoman at a department store will measure you for a bra, or you can do it at home or ask your mother or older sister for help. Use a tape measure. To determine chest size, run the tape measure under the breasts and around the back. Hold it firm, but not too tight. The measurement will be in inches. Add 5 inches to that number. This is your chest size. 
For cup size, loop the tape measure firmly around your body at the fullest part of your breasts. Subtract your chest measurement from that figure. The remaining number should be between 1 inch and 4 inches. This is how you determine cup size.
Less than 1 inch is a AA bra cup size. 1 inch is an A, 2 inches a B, 3 inches a C and 4 inches a D. If you end up with an odd number, round up to the next even number. This is especially important with young girls because they are growing so fast that if you round down they won’t fit in the bra long if at all. Girls are usually ready for a bra when they have an A cup.
Learn how to actually wear a bra. Don’t be embarrassed if you need to tell your mother you have no idea how to wear a bra. Many girls need to be shown how to do this, and it's OK to ask.
To put on a bra, put your arms through the bra straps and lean forward so the breasts fall into the cups. Fasten the back in the middle hook (training and sports bras don’t have anything to fasten, which also make them good choices for a very first bra.)
Adjust the straps if necessary and refasten the back of it on a different clip to change the size.
You could ask your mother to take you to a department store and get you measured and fitted for your first bra.Some mothers try to make it a fun mother-and-daughter day.