You can't until you will either:
- get promoted by someone;
- gather your first badge by commenting;
- receive a badge from someone.
You see that thing next to your nickname at the top bar (one where's all the navigation)? Counter goes up to eight small dots, then resets and you get your first badge to give for people's contributions. It's a slow process but you'll get there. Afterwards, you will also be able to promote others and generally have all of the privileges. Until then, you are left with commenting on the existing topics.
Why is that? Hubski was plagued by spammers of various kinds and it's a solution to curb the problem. Nothing personal, but it's something that was direly required.
If you have any other questions, then please don't hesitate and ask. However, I would advise you to read both the FAQ and the Hubski Primer. These should cover almost all of the common questions.