How to install a custom rom on my galaxy s3 mini?

Hi all,
i would like to ask how to install a custom rom onto my samsung galaxy s3 mini. I know there are many guides but i am a complete starter, i just made a switch from apple and i have no idea what they mean, even the clearest one:confused::confused:.. I just want to update these devices to lollipop since they are still on 4.1.2 jelly bean. Omnirom or cyanogen or others, i dont mind, i just want an updated device. Step by step instructions on everything would be perfect thank you!! By the way, i dont have windows, only mac.
this would help me a lot in the future.
thanks