Sample based on 2 stratas/variables

Hello guys,

I have a dataset of 90 observations (health center types). I only need 30% of my cases. I have two stratas/variables (I guess): Region and Hospital type. There are 10 US regions and 5 health center types such as family planning, state dept. etc. How can I do this? what would be the command. I know the command is sample or bsaample. But what would be the logic Please I will appreciate any help.