Of course, reading the guide is necessary, such as How to start a public job on Translia -> http://www.translia.com/node/22. But, I'd like to give some tips for a quick start. It's much easier to start a job on Translia than you ever thought:

Log in as Translator;
Find a job and open it;
Read the instruction and project info under View tab;
Click Translation Tab to translate online segment by segment, and update segment status accordingly;
That's done!

And, take the quiz (http://www.translia.com/quiz) at the top right would be helpful too!
Finally, why not have a try on the free jobs of your language? even only one sentence! Good for you and help others.

Why I can't do translation? The possible reasons:
1. The job is closed. Translator can only work on Open jobs.
2. The target language is not your native language. You have limited access to such jobs.
3. You need to boost your Translator Rank. If you can't edit the segment which is set to Ready to commit in an open job, it means that a higher Translator Rank is required. Only translators with rank higher than 7 can edit such translation if they find something can be improved.
Translia feature
As to the job notification, Translia is quite different with other websites that you may work with. For them,

You get the job notification, then contact the client or provide your quote, wait for the client's decision and may get the job after email communication with the clients.

On Translia, once you get the job notification, it means that you can start translation immediately. You have the source content, word count, deadline, total rate for the job...