You can't. Your mind will wander. Just acknowledge that your mind has wandered and return to your meditation. This will happen many times in a meditation session and is part of the process.
Always come back to noticing your breath, not trying to fight thoughts but rather accepting them and letting them go.
The best way to keep your mind focused is to concentrate on the breathing and then if any physical sensation that occurs, Focus on that. If you start the meditation and feel you can't concentrate because of a tight chest etc. Focus on the tight chest. The breath is just an anchor to keep you grounded from thinking, but using the sensations adds a new dimension to keep it fresh and keep you curious.
You can't actually keep your mind focused. Your goal is to simply observe the movements of the mind. Then you might gradually just learn to see that the movements of the mind aren't YOU. And it's unnecessary to identify with those movements.
Try taking deep breaths and cocentrate on your breathing for as long as it takes to clear the mind then go back to your natural breath and relax