new wireless network security and sharing question

ok i just bought a wireless router d-link di-524 52/per second blah blah blah...got it up and running just fine the problem isi dont know how to set it up so i can share files from my computer upstairs that is hooked into a wlan line and the one downstairs that is connecting through the wireless part.

second question: how do i set it up so others cant drive by and use my network connects (ie. isp and printer and hack my shared files...)

i have windows xp one computer is a very new dell 3.2 with 1gb mem ect ect and the other is also xp but a p3 550 with 700 something memory.

any help or good web pages i can head to would be great.

 

i guess the major problem is i am not sure i even got a 'home network' set up cause i created the home office thingie in the network connections and it messed up my connection downstairs where it lost the ip info and made the wireless network stop working...ahh more things to do...