I am using Metamask for testing my contract which requires some ether, but, I don't have any ether in those accounts. I know Remix which provides four accounts with 100 eth. I have no idea how to get ether in Metamask.
Metamask is a client and basing on which network you will connect it would give you the respective balance for that account.
Basically, if you want to test your contract you need to run a node locally or, better, you can use a testing RPC client for like ganache-cli/testrpc.
To connect metamask with your testing rpc client you can follow this guide http://truffleframework.com/docs/advanced/truffle-with-metamask
In metamask, you can connect to Ropsten network and click on 'Buy' and the 'Ropsten test faucet'. This will take you to
Now, under the user section, you'll see your address. From metamask, you can change your account and the address will change. In the above link, click on' Request 1 ether from faucet' and this will generation a transaction. Once the txn is complete, your balance will be updated.
Don't try this too often because you may be 'greylisted'