To maintain a good merit history you have have to keep making quality posts at all time it will help to increase your rank but i have an advice for you, as a newbie merit shouldn’t be priority at the moment you should seek knowledge first it will help you in making quality posts and useful contributions that will earn you merit. Focus on learning and contributing positively to the forum and the merits will come eventually.
That's the point most newbies don't understand or they simply don't understand this due to a lack of knowledge about the system. They hear from a friend or someone that it is possible to earn money from this forum but you need to have merits for that, and as soon as they create an account, they start a topic asking about how they can earn merit and what are the requirements, etc.
Most of such newbies, after learning the details about the system and how they can earn merits, either abandon their accounts and leave immediately or give up after trying for some time because they aren't able to make constructive contributions in the forum and that is the only way they can grow over time. You can't expect to get merits and grow in ranks if you are making useless posts all around the forum no matter for how long you do that.