Messages cannot be sent until you have posted a little more, same with editing anything. When you are ready and are able, just hit on the person's avatar (little picture with their name) and underneath you will see 'start a conversation'. When you are finished with your message you hit start a conversation again. I'll flag your question so a moderator can help you with further detail.
Hi edu pintilie
and welcome to TCS
Private messaging is limited to members with at least 5 posts. We do that so we can get to know them a little bit before giving them this particular privilege. Once you are able to PM members, please use that option with care. Avoid suggesting products, services, or sites - doing that could get you banned for spam.