The alphabet is a very abstract concept, and an especially hard one for a first grader, I think.
If you want to use the alphabet to help her, you could have an alphabet strip at her workspace, and you could refer to it as you're helping her look up words. You could think about this as similar to using a number line for solving addition and subtraction questions. For example, if you're looking up a word that starts with the letter "c", you could look at the alphabet line, and she'll see that she needs to look at the beginning of the dictionary. Etc.
You could make a letter tab on the side of the page for each new letter. That way, she could figure out the beginning letter of the word she's looking up, and then look for that letter tab.
A children's thesaurus might be more helpful for her. She could then look up topics, like "animals", or "cats", etc.
If these are all too difficult for her, then you could look up the words, and she could watch how you do it. You could tell her what you're doing, as you do it, so that she's aware of the thinking that's involved in the process.
Wow! I'm so impressed that you're even thinking about the dictionary! My son is in kindergarten and I can't imagine that a year from now we'll be to that point. I wonder if maybe using a children's dictionary would make the process easier and more fun for your child. I looked on amazon.com and there are lots of childrens' dictionaries to choose from.
If you end up getting one let me know if it works!
You may want to consider another approach. Our children are growing up in an era of high technology. Introduce your child to looking up words by using the computer. You will still teach the alphabet, but also help your child become familiar with the computer, which is essential for today's living.
Shirley at www.blendedfamilyadvice.com
Kids love using the computer. You can have her use dictionary.com on your home computer. There is a child's version as well that has animated pictures and easy descriptions. If she doesn't read well enough yet, you can have her type in the word and you can read the definition out loud. Great, fun activity to do together. You can also always use the "I don't know what this word means, lets look it up and learn the definition together."
This is just a strange question unless your child can read. Are you saying she is a good reader already? In which case your kid is a genius and maybe you should let her evolve on her own. She'll know when she needs a dictionary.
Children recognize more than 10,000 words in first grade. Maybe 20,000. Alright, she sounds out a word and know it! Bingo, no need for a dictionary. If she doesn't know it, that means she's reading books with a fairly advanced vocabulary. Why not use more appropriate books?