I've been searching around for quite a while for the same answer and while most videos and tutorials try to explain Steemit in general, they did not dive in to answer what this post is really asking about - which is "How to buy and how to deposit steem into steemit". This is from the perspective of someone, like me, who bought STEEM in an exchange and planning to HODL it, and while doing so, convert them into "Steem Power" which is useful for voting, etc. instead of sitting in the exchange or a wallet somewhere.
Actually there are lots of information out there, but just buried deep in all the general tutorials. Finally, found the answer in this post https://steemit.com/howto/@cryptotrendz/how-to-transfer-steem-from-bittrex-to-steemit
To buy Steem, get it from any exchange using Bitcoin, ETH, etc,
If you don't like to read, watch this video:
I also found this video giving a good idea of how $$ relates to wallet
Hope it save others like me some time, who come to this post as first stop ;)