Whats the best way to warm up your voice to sing to the best of your ability?

sometimes i sing GREAT. sometimes its really off, and cracks, and i cant get up high. i want to know how to warm up, and also if you can please tell me the best way to hit a high note. and im about to make a cover for a song named Next 2 you by justin beiber and chris brown..its amazing...you should totally look it up and i wanna sing like a angel... so yeah ? (: