Now that you already know the correct posture to sit down and meditate, once you're observing your breathing, stop just for a few moments to observe your mind and realize how thoughts arise from it like bubbles without you even trying.
Try and let those thoughts through your mind; as they come, they go. Really try not to anchor yourself to any of them and realize how they come up unceasingly. Limit yourself to observe, no judgments involved.
Practice this exercise for 5 minutes.
TIP: Before sitting down to meditate, play some relaxed and calm music. Preferably choose a melody you're not familiar with to avoid distractions.