Being a motorcyclist is probably one of the most dangerous road-going hobbies on the planet.
You can be the most skilled experienced biker and still get knocked off your bike by some irresponsible van driver or even by a less experienced biker! The only thing you can do is ensure you are as safe as possible when you are out and about on your bike and having a decent helmet is the first place to start.
You should always buy your helmet brand new - if you are looking at second hand ones then STOP as you don't know what the helmet has experienced or if it's been used in a crash or anything.
You should always go and try your bike helmet on before you buy - or buy one you have tried on before and you know it fits you well.
Having an incorrectly fitting helmet is as dangerous as not wearing one at all.
In addition to these obvious things, it's also important to never draw or write on your helmet with any permanent markers, or stick stickers on them - as the glues and solvents used in such products can weaken the helmets - and we don't want that do we! There are places you can go, mainly professional body shops - that will professionally paint your helmet - they use water based paints and techniques that will in no way weaken your helmet, so if you are insistent in having it modified then please consult the professionals.
To summarise, make sure your motorcycle helmets the right size, brand new, and un-modified.