Q1) Go into your router's config page, and look for wireless or security section. Make sure to use wpa or wpa2 pre-shared key (more secure than wep). Use a long password (i use a key generator and save it to a text file and copy the file to the other computer).
Here's an example: [img=http://img60.imageshack.us/img60/8004/netsec7kw.th.png]
Q2) First, make the workgroup names identical. Then, enable sharing on things you want to share.
Files: Right-click a file/folder and then click sharing and security. Make sure you allow users to change network files.