Sure; basically, Mormons believe that God was once mortal like we are, but for whatever reason is now the supreme being and creator of the universe. Because God was once like man (meaning humanity, not the gender identity), we believe that we can someday become like God if we are righteous enough, and are married to a similarly righteous person in an LDS temple (though there are a few exceptions to this rule, including for those who didn't have the opportunity to hear the LDS gospel). This is NOT a thing that all Latter-Day Saints get.
The part where "planets' come in is the idea that because we can become like God (though specifically
not worshiped like God), we'll have the same creative, cosmic power as God himself, and so we'll able to organize matter on planets just as God does, and perhaps to create life itself on those planets. The misconception that non-Mormons have is that every Mormons gets a planet, but in LDS doctrine it's merely assumed that the most righteous of the faithful have the opportunity to do what God does and organize planets, matter, and life.
And the "creating planets" bit is really a smaller part of the idea that God will teach the righteous just how the universe works, and will let us explore the wonders of creation.
Do the LDS have a book that explains what they believe? I mean something like the Catholic Catechism. It's very hard for non-lds like me to tell misinformation from what they actually believe.
Yes, it's called the Book of Mormon, the Pearl of Great Price, and the Doctrine and Covenants. We don't have any short Catechism beyond the very basic
Articles of Faith.
