You can talk to me! Ask if they want to go get coffee or something and just tell them in person. Just tell them the truth. It takes more than a simple conversation to show someone that your truly mature i think.
As long as it's not "love at first sight" or "love after a great first fuck" I don't think you have to worry about how mature it's going to sound...But if you need some tips,well yeah,honesty is the only way and being calm and reasonable helps alot.Just make sure the person's worth it!