There are commercial diets available for mice that you can use. There are two types of commercial diets: seed mixes and lab blocks. Seed mixes have the advantage that you give variety to your mice, which they like. The disadvantage to seed mixes is that they pick out the seeds that they like and don't eat the others, and to have a balanced diet that provide them with all the essential nutrients they need to be healthy, they should eat everything in the mix. Lab blocks have the advantage that they can't pick out what they like, being presented in a pelleted form with all they need. The disadvantage is that this diet can get monotonous for mice, who like variety. Also, remember that you should supplement the diet that you choose with fresh or dried fruits and veggies. Beside their regular diet, my mice get apple, carrots, grapes, raisins, different types of dried fruits, banana, ect. Also, you can offer them cereals like Cherios or Rice Krispies (avoid cereals with sugar added). Mice are omnivorous, so a bit of cat or ferret food, or a tiny piece of chicken can be offered every now and then.
 Remember that your mice should always have fresh water available. This can be provided in a dish or a water bottle. A water dish usually ends up covered in litter, so it is way better to get a bottle for them.