According to some references, it is said that the Earth's inner most layer gets heated up and exists in a solid form due to Earth's pressure. But how does that inner-most core get heated?
The inner core is not getting heated up - it's cooling. Much of the heat was released during the formation of the Earth, and the Earth as a whole is slowly losing that heat from its surface. Although a lot of heat is generated from radioactive decay, this only slows down the cooling. The entire core was once liquid, but as it cooled the center started to solidify and the solid inner core has been growing ever since. See, for example, Earth as an Evolving Planetary System by Kent Condie (pages 145-146) and The Inner core of the Earth by Don Anderson.