The first heat can happen anytime between 6 months to 2 years - it really depends on the lines and the individual. You will notice a swelling of the vulva, and then a discharge which shortly becomes bloody. Heats last 3-4 weeks, and she can be fertile really anytime during that period (more commonly day 10+, but there have been breedings that have occurred earlier than that - especially since bitches tend to keep themselves cleaned up, so you might not notice the discharge for a couple of days). She may get moody and/or clingy, and refuse meals or vomit.
If you are spaying her, now would be the time - there is a dramatic decrease in the chance of her contracting mammary cancer if she is spayed before her first heat. If you are not spaying her, she needs to be on-leash and supervised whenever she is outside - even in a fenced yard. Bitches can and will jump fences to be bred, and it can happen in the blink of an eye. Dogs can mate through chain-link fences, and as we've seen on this board will go to great lengths to get to a bitch in heat (refernce the thread where the Bordeaux puppy ate through a door to breed a Boxer bitch).
If you are walking her, take care to avoid any dogs, especially unleashed ones, you encounter. It would also be courteous to take along a large bottle of water, and thoroughly saturate any areas where she urinates - this will keep the male dogs from swarming the region. Season recur every 6-12 months, so you will have to go through this 3 or 4 times before she is able to be bred.
You can get special "panties" and cotton pads for her to wear in the house, which prevents a large mess, but take them off before she goes into her crate or at least for several hours throughout the day - she needs to "air out," so to speak. Cover her favorite sleeping spots with an old blanket that you won't mind getting blood all over.
Once she is out of season, you will have to watch her closely for signs of pyometra (a life-threatening infection of the uterus). This occurs in unbred bitches anywhere from 3-12 weeks after a season. She will also go through a false pregnancy - the intensity varies, from showing no noticeable symptoms to adopting a toy to actually producing milk.
