You don't need to block your thoughts.
Meditation isn't about emptying your mind. It's about learning to observe your thoughts, without getting too involved in them.
Don't worry if you fall asleep.
It's more common than you think. Don't chastise yourself, and don't think you've 'failed'. It happens, don't dwell on it. Top up on your sleep and try again tomorrow.
The more you practice the easier it will be.
The act of slowing down, observing your thoughts and using your imagination to understand meditation techniques will feel alien at first. But try and practice it on a regular basis and things will start to flow better - plus you'll feel tangible benefits.