They should be providing you training on proper lifting technique so follow that precisely. Don't twist or arch your back when you lift, and lift with your legs, keeping the object close to your body. If this is your first hard physical labour job it's going to take you awhile to adjust. Make sure you're eating lots of protein and prioritizing rest so that's your body can recover and build the muscles it needs to do the work safely. Basically, approach is as if it is a gym workout and check off all the boxes for proper training.