One of the most straight forward basic doctrines I feel the Book of Mormon teaches is Faith in Jesus Christ. Mosiah 4:8-10 reads the following;
8 And this is the means whereby salvation cometh. And there is none other salvation save this which hath been spoken of; neither are there any conditions whereby man can be saved except the conditions which I have told you.
9 Believe in God; believe that he is, and that he created all things, both in heaven and in earth; believe that he has all wisdom, and all power, both in heaven and in earth; believe that man doth not comprehend all the things which the Lord can comprehend.
10 And again, believe that ye must repent of your sins and forsake them, and humble yourselves before God; and ask in sincerity of heart that he would forgive you; and now, if you believe all these things see that ye do them.