Ask the guy to join you in a casual, quiet and relatively private setting, such as a cafe or in your home. Do this only after you have spent enough time with him to know you are interested in being more than just friends. If you have only met on a few brief occasions, it may be better to spend more time together before confessing any strong feelings.
Begin with a casual conversation rather than jumping straight into anything more serious. Ask him how his day is going and what he's been up to recently. Do your best to keep the atmosphere laid-back by making jokes and small talk. Do this for a couple minutes before moving on to your main topic.
Transition from the less serious conversation by saying something like "I've been wanting to tell you something." As you say this, stay confident and remember that you will never get what you want by saying nothing.
Explain your feelings in as straightforward a manner as possible. Tell him why you enjoy spending time with him and explain how he makes you feel. Mention qualities that you like about him. Finally, discuss how you would like your relationship to progress. For example, ask him if he would like to go on a date some time or, if you have already been seeing each other casually, ask him if he would like to be exclusive. Be prepared for whatever answer he may give.