I don't use the sharing buttons much, but then it's easy to copy the link.
Reputation starts at 25, but will go up quickly after a few votes. I'm on 65.9.. and climbing very slowly as it's a logarithmic sort of thing. If you get flagged at the start it could easily drop to negative. That's how spammers are dealt with as their posts become invisible by default and they are likely to give up unless they want to pay in a lot of money to boost their SP. You can see detailed graphs at http://steemwhales.com/andyc when it's picked up your account.
I'm not sure about voting on your own posts automatically. I think it deprives others of a share of the curation rewards. The post value is estimated from the payout that will be split between the 3 types of Steem (unless you choose 100%SP). https://steemstats.com gives a breakdown of what you are likely to get and also shows which votes contributed to it.
I'd hope that tag completion will come in time. The site is still in beta and more changes are planned. It's a work in progress and I'm not sure it's ready for general users, although there are some non-techie people here.
If you want images they have to be hosted elsewhere. I use Dropbox and the cool Keybase file system. I have a script to do the latter for me. If I want one from Google Photos you can get the image link by right clicking whilst viewing it. It's a ridiculously long URL, but it works.
Thanks Steve. Very helpful. Much of this is probably in the Steemit User Guide which I havent bothered to read :-)
I'm not sure about voting on your own posts automatically To be clear Steve, that (automatically voting on your own posts) appears to be the default behaviour unless it's a privilege to help newbies out.
I must have turned it off. I don't get all the design choices and don't claim to understand all the finance ins and outs. There's a FAQ and Welcome page in the menu on the right that may be worth reading.