Mitosis is an asexual form of cellular division within somatic (body) cells (as opposed to meiosis, which is the form of cellular division of sex cells). Prophase is the first step of mitosis. The other steps, in order, are: metaphase, anaphase, and telophase (the acronym PMAT is used to remember the steps of mitosis).
Prior to prophase, each chromosome makes (replicates) an exact copy of itself, thicken, and coil. During early prophase, the centrioles divide and move towards opposite poles of the cell. Asters begin to form from the centrioles. Meanwhile, the nuclear membrane beings to disintegrate. During late prophase, the centrioles and asters arrive at the opposite poles of the cell and the nuclear membrane has almost disappeared.
The below link provides a much more detailed explanation of each stage of mitosis.