There are plenty of step by step guides about this on the web, but I can't point you to any until we know exactly which flavours of Windows you have. The mention of Homegroups suggests Windows 7 - are both PCs the same? If not, Homegroups are not an option.
The first thing you need to do is make sure that both PCs are part of the same workgroup. Typical workgroups names will be 'workgroup' or 'mshome'. It doesn't really matter what the work group name is, providing the router and both PCs all belong to the same one.
For printer sharing, right-click on the locally-installed printer and choose Printer Properties. In there, you will likely find a 'Share' tab where you can enable printer sharing, and can name your shared printer. On the other PC, add a new printer, choose Network Printer, and then browse for this printer. Alternatively, you can specify the printer directly: '\PC1\MyPrinter\'. Windows will try to download and install appropriate drivers for this printer. If you have any problems with this, you can explicitly download and install the drivers for your printer, BEFORE you try to connect to the networked printer.
For file-sharing, you need to decide what you want to achieve. Is one PC the master PC, thus you want create a shared folder in this? Or do you want a shared folder on each PC?
In Vista and Win7, you have the option of Public Folders, but you can always explicitly share specific folders. Rt-click on a folder, choose properties, and the go to the Share tab to enable sharing on that folder.
An alternative, if you have a good web connection, is to use an online service, such as Dropbox, to create a single resource that you can access from both PCs and that doesn't depend on one or the other PC being switched on. In addition, the same benefit can be achieved by buying a NAS box (a networked hard disk, in effect).
Before I can give any more help, I need more specific information, and using the information above, I need to know what you have tried so far, and how far you got.
How to attach to a homegroup...