You can register by simply place a new ad - See How to place an ad? - and complete the process after you finish posting your ad.
Or you can follow these steps if you don't currently have an ad to post.
- Click on My account.
- Click on Register.
- Supply the info required such as your email and choose a password
- Check your email for a confirmation email and click on the link inside it.
- You are now a registered user on OLX!