Birth control question! Help!!!

Ok so I know that when you start birth control you may. Have to try more then one so you can find ones that fit your body for lack of better words. My question is when ppl say "it didn't work" does that mean it made your hormones out of whack? Does every birth control stop you from getting pregnant if you take it right? I'm asking bc after I start my birth control and I'm on it for a month I plane on having sex with no condom is it safe ?