Are all cells similar in cell and structure? If yes then are brain cells and heart cells of the same kind? Then how are tissues formed according to the functions of the cell?

No, all cells are not similar in structure and function. Cell structure is based on the function of cell. No brain cells and heart cells are not of same kind. Brain is made up of neurons and heart is made up of muscle fibre cells. Both are structurally and functionally different.  
Tissue are divided based on the structure and function of cells. For example, nervous tissue cells are narrow and long that helps them to carry impulse easily and efficiently.