The most daunting stage for many is writing the first message. Most people limit themselves to a simple "Hi" or "How are you?". However, such messages get lost among dozens of others and rarely spark a desire to give a detailed reply.
For your first step to be successful, show that you are genuinely interested in the person. Carefully read their profile and look at their photos. Find a detail to hook onto. For example, if there's a photo in the mountains, write: "Hi! Great photo in the mountains. Is that local or somewhere abroad?". This shows your attentiveness and immediately sets a topic for conversation.
Try to ask open-ended questions that can't be answered with a simple "yes" or "no". This encourages the other person to give a full response. Remember that the main thing is to be polite, sincere, and not pushy. Don't get upset if someone doesn't reply; just move on, because your person will definitely respond.