There are two scenarios - if the woman is ovulating, or she is not.  If she is not ovulating, then the sperm, would die its natural death after a few days, but if she is ovulating, the ovum released by her would be in the fallopian tube waiting for the sperm to fertilize it. Out of the millions of sperms which had begun their journey only a few capable sperms (few hundred) survive to reach up to the fallopian tube. 

If we look at the anatomy of a sperm, The sperm has a head and a tail. The head is covered by a cap called acrosome. The acrosome contains enzymes which help in the dissolution of the wall of the ovum.  Below the head, the sperm has a few mitochondria, which provide energy for it to swim in the women's body. The sperm can cover a distance of one centimeter by lashing its tails a thousand times. The distance it has to cover from vagina to the fallopian tube is roughly 15 to 18 cm. So it could take half an hour or sometimes several hours and sometimes days for it to cover the distance depending on the strength and health of the sperm. Some sperms could get stuck in the folds of the cervix or uterus and may not reach the fallopian tube. If the women have an infection, then she may be producing white blood cells, which in turn could destroy the sperms too.

Once the 100 odd sperm manages to reach the ovum, breaking all the barriers in their way,  the next part of the journey starts. This is the most difficult part - Penetrating the wall of the ovum to reach the nucleus of the egg. The sperms drill through their head and try to penetrate the wall. The enzymes in the acrosome are now released and this helps in dissolving the wall of the ovum. The sperms have to penetrate one cell layer after an another to reach the innermost layer of the ovum.  Several of them perish on the way.  Finally, one sperm manages to drill its way into the plasma of the ovum. Once a sperm is inside the ovum, Immediately, the ovum shuts itself, so other sperms do not enter it anymore. The head of the sperm burrows inside the ovum and the tail is shed outside. The head of the sperm with the genetic blueprint approaches the genetic material of the woman stored in the nucleus of the ovum and fuses with it and thus begins a new life.

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