Oh OSX servers are horrible stick with either Windows or Linux, Apple is simply awful in the server department.
I have got 3 servers myself and have set them all up from teh ground up and it isn't easy to learn how to do it you will find many issues along the way here are my top tips.
1) Start on your local PC, servers are expensive and you can run a server on any Windows XP Home or later OS using IIS.
2) As for opening ports on your router you should not have to do this most routers come with port 80 open already and that is the port that is used for web servers it is safe for you to run a server with port 80 open. If you do start heading into opening other ports maybe for a mail server you can usually be safe doing it so long as you have a firewall.
3) Read many tutorials before even starting you will find them invaluable in setting up the server.
4) Invest in a domain name with a host that gives Free DNS management this will mean that you can concentrate on your server and not with the hard stuff to do with DNS and setting up a DNS server.
5) I suggest for a beginner that you stick with a Windows OS, Linux is great and i have a server using it but if your used to Windows making the leap to Linux can be terribly hard.
6) I 100% do not suggest you buy a server and host it at home it is a waste of money, ISP for normal households do not give you the speeds required to run a server and will throttle you back if they see extensive port 80 usage. You will waste your money buying the server when your ISP slows you down. I suggest using a reliable data center like i do and host your server in their facility this will give you large data allowances and is very cheap you will find.
7) when buying a server get the right OS for the job don't buy a PC with the intention of using it as a server and buy a normal windows package get Windows Server 2003 or 2008 they can be costly but you will appreciate the advantages.
8) Always run a server with 64bit if you can support it with your hardware.
9) Good Luck and if you want help email me.
This was typed fast so please excuse any mistakes or bad grammar.
EDIT
10) Another down side of hosting a server in your home is you do not have power redundancy, no network redundancy and no fire protection.
Mystic Mod - Putting the fear back into sliced bread since 4th May 2010
Windows 7 Pro, Intel Core 2 Duo 2.8Ghz, 4GB DDR3, Dual nvidia SLI 9800GT 1GB, 1.5TB Hard disk