The translations do not differ in doctrine! Except if a cult like the Jehovahs' Witnesses have re-written it and wrongly translated it to fit their doctrines.
The differences start with different word usage, depending upon when the Bibles were translated. So the KJV has 16th century Engish. If you like Shakespeare that would be a good choice! I never liked Shakespeare, and the different words and grammar make it so hard for me to read. I was raised with the KJV but I had to read a modern version to really understand the message of the Bible. To me, the words are just too archaic to read, although many people love the KJV.
The NASB is a translation from earlier in the 20th century and is dated in some places. I read it for 20 years, till the Bible wore out. The NIV and ESV are more modern, plus the newer versions have been revised as scholarship improves.
Another difference is that different manuscripts were used to translate. KJV used later manuscripts which had a lot of additions and mistakes when compared to to the earlier manuscripts which were closer to when the original autographs were written. In 400 years, a lot of ancient and early manuscripts were discovered, which shed light on many textual issues in the KJV. But although there are differences, with the exception of the longer ending of Mark, these differences are more about a letter that was missed or added or changed in a word. Lower textual criticism of the Bible has tracked ALL of the extant manuscripts, and each copyist mistake can be followed in a chain. The Dead Sea Scrolls are also an amazing example of the preservation of God's Word. Hebrew texts, including an almost identical copy of Isaiah are among the scrolls. They date from before the time of Christ, and show that God has preserved his Word.
Further, there are choices to be made about how to translate. Are you going to translate "word for word?" As my Greek prof has pointed out on many occasions, Greek is very different than English, and a true word for word translation is not possible. However, the translators try to follow the words as closely as possible, without making the English into bad English. Plus Greek has many different verb tenses and noun cases than English, so that needs somehow be conveyed. That becomes the choice of the translator. Hebrew has very different grammar than English, being a Semitic language, but it does follow. Word for word translations are known as Formal Equivalence, and includes KJV, NKJV, NASB, RSV, and ESV
Or the translator or translation committee can choose to go "thought for thought." In other words, rather than just try to woodenly translate, they put the thought the Biblical writer was trying to convey. It makes for much smoother English. This is known as Functional Equivalence, or dynamic. It includes Bibles like the NIV, NAB, TNIV, GNB, JB, HCSB and NLT. (New Living Translation NLT, might be a good choice. Go to Biblegateway.com to read some passages in it.)
The last category of Bible translations is Free translations. They are basically a paraphrase and not a good choice for beginners. The Living Bible is one example. I remember reading it one year. I got to a place where something was not ringing a bell, but it sounded very appealing. I compared it to my other bibles, and it literally was off beyond left field. It was quite simply wrong. The NEB and the Message are two other examples of Free translation.
The best theory of translation is one that remains as faithful as possible to both the original and receptor language (English for example) But when something has to "give" it should be in favour of the receptor language -without owing the meaning of the original language, since the very reason for translations is to make these ancient texts accessible to the English speaking person who does not know the original languages.
Finally, as a speaker of a foreign language, you must be aware that you cannot exactly translate a word on many occasions. Or there are several alternatives to a word, so different versions might choose a different word. Every word of the Bible has been tracked for centuries, and tools like Strong's concordance and Bauer's Lexicon are available to check word choices.
I hope you will let us know which translation you choose to go with!