Breath Awareness

im on my first week and im really confused ,when im doing breath awareness what kind of sensation im looking for i mean how do i know that its working and im doing it right ,also is it a problem if you space out i noticed that while im concentrating i only do it for a short time then i space out or a thought coming distracting me . i have a last a question but when should i exercice ,should i do it before going to bed or i will be too tired .