Every 25 posts, also including topics you make, you gain 1 rep power.
Every 100 rep points you receive from users, you gain 1 rep power.
Giving rep to users doesn't take away from your own rep.
Stories_Archive.html
^^^ Posting here does not give you any rep points or power even if you post 25 times, however it will count towards your overall post count.

Adding to what TaigaAisaka said, if you click on your name, scroll to the part where you see your friends, then at the right side of that bar there is a tab named "Reputation". Click on that and it will tell you all you need to know.

You will see a template like this:


The Tech Game Reputation System

In the last 24 hours, you have given reputation to X poster(s).
You can give reputation to 10 more poster(s) today.
You need X posts to earn 1 Reputation Power.
You currently have X Reputation Point(s).
You need X more Reputation Points to earn 1 Reputation Power.

Then below this you will see all the rep points you gave out, and all the rep points people gave to you. Again, every 25 posts = 1 rep power and every 100 rep points = 1 rep power. The only way to get rep points is by people giving them to you.