Port forwading - I have 2 routers, but I don't know the IP of the second one?

First of all, you have to identify how your network is connected. You have your primary internet gateway, which is your DSL Modem, Cable Modem, etc. Etc… Then it is connected to lets say your new router, then your old router is connected to the new router and your computer is connected to that router. I do not know how your network is connected for real I'm just assuming.

The first router lets call it your primary router will have the address of 192.168.1.1 and it is probably set to DHCP which means that it lets the router choose what address to give all the other devices, now your second router might have the address of 192.168.1.2, to access that router you would have to put in that address in your browser. Now all of that is just to give you an idea of how your network is setup.

Go into the primary router configuration menu and find the option Attached devices, then use that to help draw you some kind of map to get an idea of how your network is connected, write down the relevant ip addresses, then go into the secondary router and do the same. Once you have identified all the pertinent ip addresses and have mentally and visually mapped out your network, then your are ready for port forwarding. Go into your primary routers port forwarding option select the protocol you are going to forward and forward it to your secondary routers ip address, then go into your secondary router and do the same only forward it to your computers ip address.

You can make it a lot simpler by simply connecting your computer to the primary router if you can, or make your old router the primary router and then you don't have to worry about the new router.