TTN will seem rather empty when you first log in, but it will start filling up as you make new friends. Their posts will show on your timeline and yours on theirs.
The fastest way to find new friends is to click on the search icon in the top menu bar and type in the name of your friend or a Group that you are looking for.
You can also go to the Groups page by clicking on the Groups link under the Community section of the menu and then browse the Suggested Groups. Each time your refresh the page new suggestions will show up.
If you do not find a Group that suits you then go ahead and create a new Group. You can find a video on how to do it here below.