[Squeaky]

I can go to some IT-forum but there must be some guys here who can help me out.

I want to create a network with two routers. What I have right now is:

1 modem/router which I use for wireless Internet downstairs. This router is connected with an Ethernet cable to my tv/mediplayer and with a cable to my desktop upstairs.

I received a new modem with built in router so I have a spare router. My wireless reception upstairs sucks so I can use that extra router upstairs.

I replaced my modem with the new modem/router and I placed my old router next to my desktop upstairs. I connected the modem/router through a cable with my router upstairs and connected that old router with a cable with my desktop.

So now I have a working cabled connection and two different wireless connections. But I prefer one wireless network.

I'm a noob when it comes to this so I know nothing about ip-adresses etc.

All tips are welcome!

[mr.pure]

Go somewhere that you can only connect to the second router and enter it's ip address, normally something 192.168.0.1, into a browser to hopefully gain access to it's settings.
Once in, you should somewhere have the option to extend an existing network.
I don't do pcs so can't help anymore than that but it should be doable something like the above though you might have to connect through a cable to the second router.

[ez-1]

It's going to be a bit difficult to give you exact instructions without knowing the brand and models of your modems/routers but I can try to help.
First of all I wouldn't use two modems in a same network. Mostly because modem is meant to be that one device that creates the connection and because that is pretty much the most important thing it has to do it is also usually somewhat difficult to disable. There are other things related to this such as NAT that might be difficult to turn off.
Why does this matter? Well if you cannot turn those things off then you'll have two different networks. Both might be able to connect to Interwebs but you might not be able to connect internally from one computer to another.
However if you can make that secondary modem to work only as wireless access point (and ethernet switch) then you should be good. And only thing you have to do then is to create one wireless network with same SSID (that would be the visible name for the WiFi) and also the same security settings.
Let's call your home WiFi RobertsWireless
This next part I'm not sure but I would also set both access points (AP) to use same WiFi standards. For example G and N standards and 5GHz if you only have relatively new laptops. 2.4GHz if you have iPhones or other mobile devices that don't support 5GHz frequency.
then you should have some sort of security. I would go with WPA-personal / WPA2 (pretty much all relatively new devices can handle this). Then just give your WiFi a password and make it exactly the same for both AP's.

Edit:
What Pure says might also work, but you might still run into those routing problems. Try to find a place that says repeater or extend. Then you should give the correct SSID and security parameters for that WiFi that you would like to extend.

If you cannot disable all those routing things from your secondary modem then go to your nearest Saturn (or what ever IT stores there are) and buy one access point. Apple Airport Express is quite nice but if you need ethernet switch too then something else.

I hope this helps.

[Squeaky]

Thanks guys, I'll look into it tonight.

@EZ: I only have one modem. I don't want to use my old modem. What I have is a modem with built in router and my 'old' router which I want to use as an access point. I made a picture of the situation I would like. Only the extra router is what's different from the current siutation.