How do i get my full custody of my child, his mother is dirty.?

when i have our child he is always sick and one time he had a bad rash which i questioned the mother about and she said it was a rash caused by diarrhea which my girlfriend is a nurse and said it was more so im guessing the mom dont change our child alot.our child had other injuries that im concerned about she just told me the child is a toddler is gonna get hurt occasionally. i also think our child is overweight for that age 38lbs she is 2 years old she just had a birthday this month. i really want to take her in with me and take care of her i want to be in my childs life and i just got involved i was not there for her when she grew up.