As Comet C/2012 S1 ISON approaches the Sun, water-ice in the comet's nucleus will sublimate which means that it will instantly turn from a solid to a gas, without melting into a liquid. When this happens we will have a better indication of just how spectacular comet ISON will become later in the year.
During October comet ISON and Mars follow an almost perpendicular path across the evening sky, at the beginning of the month, Comet ISON will be just under 2° from Mars. Comet ISON will be around magnitude 11 so you'll need a telescope to view it.
On the 15th of October, Comet ISON and Mars pass the bright star Regulus and will form a very close conjunction with Regulus at the bottom, Mars will be 1° above it and comet ISON 1° above Mars. By this time ISON should be around magnitude 8.2 and visible in modest binoculars.
Towards the end of the month, ISON approaches the Sun on its first pass into the inner Solar System and joins the regular visitor comet 2P/Encke. On the 27th of October, Comet Encke passes the magnitude 2.1 star Denebola by 3.5" and at the end of the month, ISON and Encke will be around 20° apart.