When you're setting up the bench press, make sure you have all five points of contact established. Then, make sure you pull the shoulder blades back and down into the bench to create stability. Keep in mind, that you're going to have a slight arch in your low back. This is okay. Just make sure you don't have a giant arch or else it will put too much pressure on your lower back.
Next, be sure to set up your hands so that they're just outside of shoulder width. Then, as long as you have a spotter, unrack the weight either by yourself or with the assistance of your spotter. Take a nice deep breath in to create stability through the abdominals and lower the bar down towards your chest allowing the elbows to flare out at about a 45 degree angle.
Lower the weight down towards your nipple line, let the bar tap on your chest briefly before pressing up nice and strong back to the top. Be sure to breathe out as you press up.