Your child may be a little more reserved and inward-looking than others, and that's okay. Many times experts describe these children as "slow to warm up," which just means they need more time to adjust to new situations. Keeping your daughter out of preschool probably isn't the answer, but you need to support her until she feels comfortable, and help her work up to the challenge of interacting with many other children. Getting used to a small, safe group could be the key to moving into larger, more active groups. You could start with a playgroup where the kids get to spend time playing with each other while the parents are still around. Or a co-op playgroup where the parents leave but it's the same few children week after week — but I'd stay with her the first few times. If she keeps to herself, don't worry — she's drinking in the experience slowly. As long as she's playing, she's fine. Once in a while, you can gently encourage her to interact with others, but let her set the pace. Most toddlers don't socialize much anyway, but spend most of their time engaged in parallel play — meaning they play near each other but are largely immersed in their own toys and games.
As far as preschool is concerned, there's no hard and fast rule about when a child should start, though most experts believe children do better in kindergarten if they have attended some kind of structured preschool-type program first. Some children are raring to zoom into a world filled with their peers at age 2, while others need the quiet of family and siblings for a while longer. Evaluate the preschools in your community with an eye to your child's temperament. If your schedule permits, a co-op may be ideal because it requires parents to put in a few hours of work at the school to keep costs down and therefore your child will get to see you at least part of the time. An alternative, which also unfortunately assumes some flexibility in your schedule, would be a part-time program so she has some quiet time at home to balance her more socially demanding preschool days. If your schedule does require that your toddler attend a full-time program, try to set some vacation time aside so she can go part-time the first couple of weeks until she feels comfortable and secure in her new surroundings.
Keep in mind that your shy child will definitely need more support than a more outgoing child on her first few days in a new program, and that that's a natural by-product of her temperament. Imagine yourself in a situation that is entirely new and alien to you — perhaps walking into a restaurant in a foreign country. This is how a child — especially a shy one — can feel stepping over the threshold of a classroom full of kids her age. Try to visit the school as often as you can before your child's first day so she feels more comfortable with her new surroundings. Ask the school if any of the children who attend live near you and arrange a few playdates. That way your child will know at least one person among the group of strangers when she first arrives.
Prepare to stay a while on the first day. Say hello to the teacher and accompany your toddler when she puts her things (coat, shoes, and her lunch) away. You may also want to introduce her to a few children milling around. If she seems uninterested in meeting new people, help her start a project or activity. Remember to always say goodbye (slipping out when you think she won't notice can backfire as she may become upset that she didn't get to hug or kiss you before you disappeared), and let her know when she can expect you or your partner to come back. And leave as soon as you say you're going; prolonged departures can feel heartbreaking for you and your child. If she becomes very upset, enlist the help of her teacher, who can take her off to join an activity in progress.
It may take your child some time before she adjusts to preschool, but soon she'll settle into the routine and form a strong bond with her teacher and her new friends. She may never be the life of the party, though, and that's fine — you, her peers, and her teachers can value her quiet, thoughtful approach to life, which will help her feel it's okay to be herself.