computer switch from wire to wireless

My desktop computer using cable ADSL2+ to log on to the internet,I got external modem Netcomm NB5PLUS4, I want switch to wireless, my computer don’t have a built-in WiFi, I want to know what is the best option :
1. Buy a wireless modem router or
2. Buy a wireless router

Can somebody suggest me what to buy and what brand/product is less problem ?

Thanks for the help.