For some reason, I cannot open the link by Buenchico but by the looks of things it will give a comprehensive review of organism resistance to antibiotics.
It is complicated, but I will try and simplify it for you ( and me;-))
Organisms (Germs, bacteria) are "born" with a gene that determines their resistance to antibiotics, if they have the drug resistant gene then so be it, they are resistant to CERTAIN antibiotics,but if they haven't got it then they are not resistant to the effects of antibiotics.
But things could change for our organisms who are killed by antibiotics.
During their life, their gene which makes them easy prey for destruction by antibiotics could mutate and make them safe from antibiotic activity........resistant.........and there are many things that can make them mutate.
Another method of antibiotic resistance is when a bacterium( resistant) is paddling down a stream and passes a bacterium ( non resistant) he can pass this resistance on to our "non resistant" chap paddling in the opposite direction....a process known as "Transference" which then has the effect of producing 2 antibiotic resistant bacteria,then so on and so forth.
Please remember that this is a simplification of quite a complicated procedure which the above link in Buenchico´s post will simplify.