Here I am wishing I had got around to writing the file sharing guide for my website, but I haven't yet. I've putting off that job for a while.
I will start by mentioning client firewalls (that's the software firewall that you have installed on each network client). I cannot emphasise the importance of configuring thse correctly for communications over the LAN, because you won't get any file sharing if you don't do this. I also advise people to make sure that they 1) have a very good, properly configurable firewall and 2) know it thoroughly. Failing that, don't have one at all.
Before going any further, have all clients switched on, logged in (ideally to the Admin account) and preferably in the same room if possible, so that you can easily respond to messages and firewall prompts. If there's a laptop in another part of the house and you can't access it because its firewall is asking for a permission to be created, you won't know about it. Also bear in mind that if you can't see one client from another when you browse the workgroup, it's not necessarily because you have done something wrong, but because your firewalls are preventing the access in a particular direction for security purposes.
Next; the File and Printer Sharing has to be enabled and configured and a workgroup must be created. This is done by running the Network Setup Wizard (*not* the Wireless version) on every client. Be careful to be consistent with the settings that you choose, particularly the workgroup name.
Now create some shares on each PC. Do this in the properties palette of your chosen folders, and be aware that certain folders cannot be shared, particularly if they reside in a higher-level folder which has privacy enabled.
Next, you need to check the My Network Places folder on each client. By default, it will auto-populate, so be aware that if you manually create shortcuts here, you will often end up with duplicates. I find it best to remove the auto-population from Folder Options. It's the first check-box on the View tab.
Sort that first and we'll do printer sharing later.