Sounds like you will want to disable Run Steam on startup.
Also disable Steam Cloud sync. Unless you want to play on multiple machines it's pointless. And may cause problems with certain (indie) games.
This can all be found in the Settings menu.
I think Steam will ask you during installation or first run if you wish to update files automatically and/or during downtime.
This is something you can always set for each game individually by going to your games list, right clicking a game, going to properties and choosing an option that matches your preferences.
Actual client updates to the Steam platform will be done automatically and invisibly, but tend to be very small.
When Steam starts, you'll get a push notification window with Steam News. This is something that I haven't been able to disable yet.
It only takes a click to close, but I'm not a fan of additional shit popping up when a program starts.
Don't forget your username and password. Write them down in a notebook or something if you have to. And don't use a 'temporary' email address (like @yourprovider.com). Stick with one of the big guys like Live, Yahoo, Gmail. Or your own domain name (if it's one you'll be hanging on to).
I used my hotmail address when Steam was first launched. I moved on from that address and the account was removed by Hotmail due to prolonged inactivity. I stopped using Steam for a while. It's a pain to recover your password in that type of situation.
I'd suggest making purchases by logging in on the steam website, rather than using the in-Steam browser.
The latter won't allow me to buy anything. The browser purchasing process is much more stable.
The only downside to Steam is that you'll be making lots of neat game purchases that will add up in the long run *looks at Garry*.
After having its problems during launch and early days, Steam has developed into a very nice platform.
I like how it allows you to stay in touch with buddies and remembers which games you own and are allowed to play. So if I recall correctly, you don't have to worry about finding game discs (although that may just be for Valve games).
Also see
http://handsofjustice.co.uk/forum/index.php?topic=490.0 for the link to the
HoJ Steam Group and
my profile.