You do not have to buy any thing, just make sure that you have some ETH left in your wallet to complete the transaction, if there is no ETH left to cover the transaction your transfer will fail and you will not get back the amount of ETH you paid for gas . Now how much ETH you should maintain in your wallet to transfer tokens and ETH successfully ? The answer is, it is not fixed amount but overall to transfer ETH is much cheaper than transferring tokens due to the nature of tokens and the need to initiate the original smart contract .  

Quick steps :

1- Open your MEW and select the token/coin you need to transfer .
2- Open ethgasstation.info and check the gas price you should use . ( if safelow and standard is close then just use safelow) ( You can adjust your gas price at the top from down arrow )
3- leave gas limit as default if you are sending ETH. But if you are sending tokens you better ask for the best gas limit . ( set the gas limit high wont hurt you the extra gas always sent back to your wallet)
4- make sure that you have enough ETH left in your wallet to cover the gas price. ( leaving 0.01 is more than enough to cover the gas) usually you will end up just paying 0.001 or even less .
5- make sure that the destination address is correct . ( or else you might lose your ETH/tokens )
6- press send and enjoy