If you take the first pill at the first day of your period the pill is immediately reliable (except for the mini-pill).
If you start during your period, but after the first day, the pill is not immediately reliable. Monophasic pills are reliable after you have taken 7 pills in a row. Bi- or triphasic pills are not reliable for 4 weeks.
If you start the pack after your period you cannot rely on it for the first four weeks, whatever brand you take. It’s only reliable as soon as you start the second pack.

As long as the pill is not reliable you must use an additional method of birth control, like condoms. You should do this anyhow if you also want to protect yourself against STD’s.