It's all written in link which Emerald posted.
But if you want to read my wall of text here is it too
You got 2 options, if you want wireless connection also from new router (advantage is better coverage and probably better signal), you have to use 2nd router as access point as is described in that link. OR if you don't need 2nd access point and wireless from 1st router is enough you actually want device called switch. I think all modern routers can act as switch too so you can use either switch or router (router give you option anytime later setup that access point if you will feel need).
Advantage of switch > no need to setup anything just plug and it work, also may be cheaper
Advantage of router > it provide extra wireless access point if you need it and if you setup it
So now how to do it:
Router you got now got 4 ports for LAN and one WAN/internet modem port. You got your modem in WAN port, now you can connect your pc or other devices either wireless or through cable to one of LAN ports.
Same goes for another router. So what you need to do is 1st turn of DHCP server on 2nd (new) router, you may need to set static IP on this router set it to same network as your main router (take it's gateway address and change last number). Next connect one of LAN ports on your new router to one of old router's LAN ports. It's important to connect LAN to LAN not LAN to WAN. This alone should be enough on most routers to make it work as switch. After that just use classic LAN cable and connect your PC to one of 6 remaining ports (3 on each router).
If you want to make also that wireless to work on 2nd router it may depend on your actual model of router. So can't exactly tell you how to setup it, but what you read in Emerald's link should be enough in most cases. Often all you need is setup that network same as on 1st router, maybe you need to tick some option like work as access point or something.