Everyone is right; just earn gems by posting. The longer your post, the more gems you will earn, but don't spam to get extra gems. Gems currently do not have much practical use; however, you can buy and sell "items" like pictures/banners/avatars/etc. in the Moneybags' Shop section. That section and the Contests DPX mentioned are other ways to earn gems, but as we said they're not very useful as of now so I wouldn't worry about them too much. =P

@MrX-Link, the shops/items thing was a feature that we planned to use early on but was never implemented. So no one can have any "items" for now.

There are actually two stores created where the owners would have been able to sell items, but as you can see they don't work.