Several infectious skin diseases also cause a rash. These can be caused by a number of infections with bacteria or viruses, and infestations of mites within the skin. Some infectious skin diseases, such as scabies, are caused by mites that bore into the skin surface causing irritation and a localised rash.
Now, as for your question about when to start taking oral contraceptives, there's really no hard-and-fast rule about when to start taking birth control pills. Starting in the first few days of your cycle is the surest way to prevent inadvertently taking pills while you are pregnant.
How long it takes birth control to become effective depends on WHEN during your cycle you take the first pill. If you take the first pill on the first day of your period (as soon as you start bleeding) you are protected right away. If you take your first pill any other time, you will be protected after taking 7 pills. If you miss any pills or take them late during that 7 days you have to start the count over.