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 mobile number and choose a password
- Check the box for "I hereby agree for processing my personal details by Terms of use"
- Check your mobile for a confirmation code.
- You are now a registered user on OLX!