with new bct update to become a full member need 100 merits and i still have only 10 merits. What is the best way to earn merits
Its very simple,post some good quality posts then it will earn you some merits. 
That is the only way you can get or earn merit.If you are getting the merits in some other ways means you are abusing the system and you will get tagged by DT.
I simply don't agree with your thinking mate. First of all we already have some people whose been doing such rules to give their sMerits, And the second one is if you did receive some merit from those people it doesn't mean that you're already abusing and abiding the rules.
Maybe there a lot of people like you who only thought that posting with high quality will be the only getting Merits. As for the reason that you're only a quality posters and not giving a goddamn idea about the things that can help other people in some other way to get some points.
How many days it took you to gain 10 merits? lets say 'x' days. Then to gain 100 you require 10x days simple LOL
Its not like what you're thinking as for me I reached my rank of Jr. Member after a month but still after a week I already became a Member and now Im still working on my posts to gain some acknowledgement from the others and currently I received 18Merits and already spent my 5sMerits.
Time doesnt give a shit about your account ranking up, it depends on how you work.