Well, I have bad news for you. And for me. You can't. Ha
I am not referring to our "internal market" exchange on our Steemit wallets.
Rather buying on the Crypto markets.
OK, what I wrote above is not true!!!
You can buy some Steem.............
But, only if you are willing to learn how to access all the exchange processes to do so.
I am in that process. I hate it. Ugh
Learning something new all the time irks me. I like simple. For a simple minded man. Ha
I had to put the Bittrex photo first. Coinbase has junk for their homepage. Maybe I don't
know how to access what they have available. Whatever. I don't want to spend hours
looking for good photos for them. It should be on the main webpage. Oh well.
Look at the below photo.
It will serve us for our puposes of this Steemit post anyway.
https://www.coinbase.com/
Some say other exchanges are just as efficient. Could be. 
xCoins.io
https://steemit.com/bitcoin/@doctorjohn/trading-experiment
Now remember. I am trying to learn by myself how to buy Steem. I want to Power Up
after I make the Steem purchase. It should be a very simple process. It isn't.
Not for me anyway. Maybe I am just making it more complicated than it really is.
This is one of the web pages I have read.
https://coincentral.com/coinbase-vs-bittrex/
It seems to have some good information. What I am learning is that I will have to
open two (2) accounts to save on costs and be able to accomplish my goals.
Here is one of our own on Steemit that has some good information. 
He suggests using "https://gdax.com" to save on Coinbase costs.
https://steemit.com/tutorial/@jerrybanfield/1-coinbase-tip-transfer-wallet-funds-to-gdax
I am trying to make sense out of all this. Yes, I can do it. But, it irks me. Ha
I am of average intelligence. At least I think so. Maybe I am wrong. lol
Opening an account at "Coinbase" is simple enough. Then the wait.
Fees for credit cards or debit cards are around 4% or near that.
If I remember right. Ugh on that.
So, the bank account fee is around 1.5% I think. So we wait....................
We will just keep smiling with our friends at "Pixabay" Ha
Thank you for learning with me. Before I die. I would like to Power Up on Steemit a little.
Can cryptos soar to the moon if you can't easily buy them?
I know. Wait a little while Francis. It takes time.............
So we learn and wait. Learn and wait. Learn and wait.
Sorry I am repeating myself. Ha ha.................................................
Francis