If you start the pull the day your period ends (because honestly, who takes the hormone pill during their period? You WANT to waste a week of freedom?) then you're fine from the get-go. If you're starting the pack anytime other than that, you want to wait a week. You'll find this information on Mayo Clinic and planned parenthood websites. Also for the few of you who have had doctors not give you straightforward answers or doctors that have changed their minds about the information they give you, find a new doctor. Find one that actually knows what they're practicing.
Also remember that if you do not take these at the same time every single day, you are putting yourself at risk for pregnancy. Planned parenthood's website has a chart for how long to wait depending on the time frames you missed taking a pill, because if you miss so many, you want to wait a certain time frame before having unprotected sex again. There are also different drugs that will make these pills ineffective, such as antibiotics. Consult a physician about whether this will work for you if you take other medications.
Another thing to keep in mind is that you want to take extra vitamins, as the pill will deplete you of things like Magnesium, Zinc, and the Vitamin B Complex. These are things that protect you against illness, depression, and weight gain. If you don't want to take these separately a good multivitamin will help.
Be aware that the pill isn't a magical anti-baby device, it's up to you to take it responsibly for it to actually work.
me & my boyfriend had unprotected sex for not even 10 seconds and that was about 2 weeks ago. now i feel kind of crampy and there was a little brown discharge & maybe a little blood but only for 2 days. i dont get my period for like another 10 days i think. can i really be pregnant? HELP!