I'd be more than happy to assist you.
1. DO NOT reset the Motorola device. If you have already done so, you'll need to call AT&T (I believe that's your provider) to have them force the settings back to the router remotely.
If you have NOT hit the reset button on the Motorola device good. Let's move to the next step.
I'm not familiar with that device in particular but it's a standard DSL router I assume so here's what you need to do.
First turn off, PC, Motorola, and Linksys devices.
First. On the rear of the Motorola, there should be a port called:
INTERNET or WAN. This is where your AT&T Internet cable connects to your Motorola Device.
Next there should be a port called LAN
or PC. Take an ethernet cable and go from the LAN/PC port on your Motorola device, to your port labeled INTERNET OR WAN on your LinkSys router.
Now it should be as simple as turning on your Motorola device. Then wait 5 minutes (since I'm not sure how long it takes to connect and be ready.
Then turn on your LinkSys router, wait 2 minutes.
Then turn on your Computer. If you're going wired from this computer, make sure there's another Ethernet cable going from your computers NIC (network interface card) to your port 1-5 on your Linksys device BEFORE turning the computer on.
If you're attemping to go wireless, then on your computer seek out the best signal and something that indicates you're wireless access point is broadcasting, then connect to that.
I don't know why you would need a wireless access point in another room if you have a WRT54G as that is a wirelss router and should work fine for multiple wireless computers. If your issue is that other room in your house is too far for the Linksys to get signal there, I suggest getting rid of the WR54G and going with a Linksys E4200 which has AWESOME range. I have a 4 level townhouse the WRT54G just couldn't make it happen. The E4200 gives me full signal effortlessly.