Making possessive of in English is not very difficult.But sometimes it does confuses. In this case, people tend to use their perception of using s in all the case.But the same rule is not applicable everywhere.So it is better to have a look on some simple rules and get rid of simply avoidable mistakes. So here are the rules.Firstly, s is used for living beings.Hence we can write Rams sister but not Mobiles charger. Instead of writing mobiles charger we will have to write The charger of mobile. And The will be used before the word preceding the preposition of.
But there are other instances where even if the noun is living being s is not used but only apostrophe . But why? This is only to avoid extra hissing sound, which even cannot be pronounced. For example, you wont write boyss school but boys school. This is because even if you add s after boys you can not pronounce the last letter s. Not only this a proper name ending in s should also be treated the same way. For example Jesus Bible not Jesuss Bible.
Lets Summarise it once more
We can not make possessive of non-living things by adding s.For this, we have to make phrase using of. So it will be The steering of car not Cars steering.
For living beings we can make possessives both ways i.e. by adding s or by making phrase using of.
 For living beings ending in letter s either due to being plural or otherwise only apostrophe is used. So, in this case, we must not use apostrophe s i.e. s
 In this case if of comes after noun then The must be used before that noun.
Hope this proves to be helpful in case of any query please feel free to ask in the comment box.