3 Nephi 11

Shortly after His visit to the New World, the Savior said to the twelve disciples he selected -

"...whoso believeth in me, and is baptized, the same shall be saved; and they are they who shall inherit the kingdom of God." (Book of Mormon 3 Nephi 11:33)

According to the Merriam-Webster dictionary, inherit means -

"...to come into possession of or receive especially as a right or divine portion..."

There are several definitions of the "Kingdom of God", but in this case, I believe the Savior is referring to the Celestial Kingdom where God dwells. Elder Bruce R. McConkie wrote -

"In the eternal worlds, the celestial kingdom is the kingdom of God, and that kingdom does not reach down to include either a terrestrial kingdom or a telestial kingdom. (D. & C. 20:29; Moses 6:57; 2 Ne. 9:18, 23; Luke 13:28-29; John 3:3-5.) The gospel is designed to prepare men for an inheritance in the celestial kingdom of God; persons can gain admittance to the lower kingdoms without obedience to the law of Christ. (D. & C. 88:16-29.)

Deity is, of course, the ruler, governor, and creator of all kingdoms of glory. Those going to terrestrial or telestial kingdoms will go to realms which are his and which he has provided. But since the whole gospel system is one of preparing men for a celestial inheritance, it is that particular part of the eternal worlds that is the kingdom of God in the true and full sense." (Mormon Doctrine)

The Savior's definition of believing in this case is to b obedient to all His laws and commandments. The essential ordinance to "inherit" the Celestial Kingdom is baptism.