It is fantastic that you are starting early in raising your child in a bilingual environment! It will be such an advantage for her later on. The process of learning to read begins with the understanding that words are made up of sounds (phonological awareness) and that sounds are represented by letters (the early understandings of phonics). Once a child understands these ideas, she can continue on to areas like sounding out letter patterns and building fluent reading skills.
If your child is more comfortable speaking in English, you may want to start by reading in English. As she becomes more comfortable speaking Spanish, she will be able to translate the principles of reading to this language. The most important thing is to continue speaking to her in both languages, and to nurture her phonemic awareness, which will give her the foundation to learn to read in any language!
Check out the Reading Rockets section on early literacy (particularly Children and Bilingualism).