The most common mistake is misquotation: letting some preacher tell you what the bible says instead of reading it yourself. The second most common mistake is assuming you know what the words mean without looking them up to verify them. You need five books to understand the bible:
1. A King James version. Other versions are ok, but all the references are indexed according to the KJV.
2. An English dictionary.
3. An Interlinear Greek-English New Testament, any brand.
4. An exhaustive concordance. Everybody uses Strong's, but I prefer Young's because it is easier to use.
5. A Hebrew lexicon if you study the old testament.