Hmm that's a big question.
Specifics of set up for security and file sharing etc will all be operating system dependent, although the principles are the same across platforms, so unless you can tell people here what that is then it will be hard for people to even suggest resources for you to look at online.
Still one thing I would say is that with web hosting so cheap I personally would not host a public website in house, especially on a server that I was learning to administer. If you mis-administer your public website hosted on your main company server, you could give people outside the building access to everything inside the building etc.
Site uptime could suffer for similar reasons, also unless you have redundant high bandwidth lines and back up power etc. you can't guarantee your site will be there and you only have yourself to yell at if it is not! If it is business critical I'd make the nuts and bolts someone else's problem until I knew the ropes.
Finally there are the bandwidth considerations, if the server and your office connections are all on the same link, there is always a conflict between company traffic for your browsing and email and users of the website, unless you have a connection that enables burstable bandwidth increases one will slow as the other gets busy.
From what you said I would suggest you firstly look at centralising storage of all your company documents then implementing a full back up process via the server, you may also be able to share applications for which you only have one licence by installing that on the server and providing single user access to each PC.
Depending on how your mail system works you could centralise email by running your own mail server to allow you to have more control, run list bots, receive faxes by email, back up messages more easily etc. But you would need to consider how you access email form outside your building if you do - and of course the same reliability /; availability issues as mentioned for the www server!
Have fun!
Memetic.