If I never give my child any junk food, will she enjoy it since she can't compare it to anything else?
Never? Never is a very long time! you need to be realistic and set a good example. Provide lots of healthy food choices and snacks for home. At parties and vacations and social occasions you may have to be a little more lax about food choices! try not to use food as a reward or punishment. Teach your child early about nutrition!
Sure. Everyone has personal favorite foods, and sometimes we don't discover those favorites until years into our adulthood. Junk food will always taste good, so kids will always gravitate that way. Your job is to limit exposure to it, so your child doesn't gain access to it on a too frequent basis. But they will always be exposed at some point.
Why worry ? Good nutrition is its own reward. A child who learns to savor quality nutrition does't give up their taste buds. If a situation arises when they are older they may judge an item on its flavor rather than some "treat & joy value" imposed on junk food by their age equivalent peers. If they like a flavor they may choose to eat it. Don't assume they are never exposed unless you are with them 24/7.