I have 2 questions in mind:
1. First,where can we buy BENs?
2. If we mine BEN through Cloud server like digital ocean-where can i find some instructions how to?
Cloud servers like digital ocean are generally for mining CPU coins, not SHA-256. SHA-256 coins pretty much require an ASIC device for mining.
You *can* mine them with a cloud server, but you will get very, very, very few BEN's with it - like maybe 1 BEN per month. Actually I'm not even sure it's possible anymore, pretty sure all the current mining programs (bgfminer, cgminer) have removed CPU support.