Here's a very simple 'noticing' exercise to try out. What do you hear when you stop everything for a few moments? Perhaps you can try this right now. Can you stop and be still for 3 minutes?
What do you hear outside yourself? Do you hear traffic? A plane? Children? People working? Birds? A bee? Wind or rain? A clock? Buzzing coming from an electronic device?
What do you hear inside yourself? Chances are that if you try to be silent for a few minutes you will notice how noisy your mind is! What is it you notice going round your mind? A conversation you had earlier? Thoughts or plans about things you need to do? A daydream about something?
This is just a noticing exercise. You don't need to do anything other than become acquainted with the noises around and inside you.
If you try this little exercise a few times it will help you grow in attentiveness. You might find a recurring theme or two in the things your mind is 'noisy' or preoccupied with. What is it you most often find yourself thinking about when you try to sit in silence?