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When you first get a pet rat, give him at least a week to get used to his new surroundings (and to you). Your rat needs to feel comfortable and safe before he will respond to training; rushing into this will not produce the results you're hoping for. Sit near your rat's cage for 5 minutes at a time, several times a day and talk softly to him. This will help him get used to your presence and voice. Once he knows you are not a threat, he will be more receptive to training. Avoid making loud sounds or sudden movements near your rat while he's adjusting to his new home. Place a hand inside his cage while holding treat and let him come to you to get it. After doing this enough times, your rat will learn that he can trust you and will also associate you with positive feelings (from treats and affection). He will also become used to coming into contact with you, which is a prerequisite for shoulder training. Use a soft voice and handle/pet him gently when he approaches you. It could take him a while to approach you at first (several minutes), but be patient. Don't give up on this because you think it's taking too long. This could teach your rat that if he waits long enough you will go away. Once your rat is comfortable with this, hold the treat just outside his cage to get him to come out and get it. Ideally, you will have him take a treat from your hand every time you approach his cage for at least the first several days after beginning this process. This reinforces the association between your presence and treats and will quickly make him want to be around you. Healthy treats that rats usually love include banana chips, plain Cheerios, sunflower seeds, plain yogurt, and peas. Be careful not to overdo it with the treats; over-feeding can lead to your rat becoming overweight, which is not good for his health. Clicker training is an effective way of teaching animals (including rats) to associate the sound from the clicker with a reward. The easily identifiable click signals to your rat that a treat is soon to come as a result of his behavior. The click sound must come immediately before the reward in order for your rat to learn this association; time this to occur just as he's about to take the treat. Your rat will learn this association most quickly if you also wait for him to do something on his own, then click and reward him immediately. This teaches your rat that he is in control of this game. Rats can understand simple voice commands, so long as you consistently use them. This trick helps when it comes time to shoulder train because you will be able to keep him near you just by calling his name. Say your rat's name whenever you hand him a treat. After doing this for a while, hold the treat away from him a little and say his name. Eventually, he will know to come to you when he hears his name, even if you are out of sight when you call him. Other simple verbal commands can be helpful too, such as "no" and "come." Work these in when appropriate.
Let your rat get used to his new home. Let your rat approach you. Hand feed your rat often. Use a clicker device. Teach your rat his name.