How to make a picture 300dpi? 10 Points!?

so i'm trying to make a t-shirt out of this drawing i made but they said that i have to send them a picture of it which has to be 300dpi in order to make the t-shirt. can i just scan my drawing onto the computer and then just make it 300dpi? how do i make a picture 300dpi and how do i check if it is? i don't even know what dpi stands for. please help, thanks.