Design a pseudorandom number generator

I am new to programming so please be gentle. I have an assignment which says "Generate a pseudorandom number generator to generate long term private keys and ephemeral keys for signing data based on SHA-I. Disregard use of existing pseudorandom number generators". Now i searched a lot and could not find any code to create my own pseudorandom generator. Is there any way to create a pseudorandom number generator myself?