Got problems with my router and its internet speed?

Hi recently joined sky broadband, they sent us a router etc, we set it up using an ethernet cable and an extension lead to the phone, connection and internet seemed fine, little sluggish, i decided to try removing the extension, putting the router by the phone and running it wirelessly, it worked a treat, speed was outstanding... however suddenly it began slowing down? dramatically? to the point where i decided to play around a little on the ip address thing on the internet, and re-setting the router... BAD MISTAKE, now its even worse! and i really need help? i dont know if i got the correct wiring set up, or if im doing something seriously wrong? ( btw there are roughly 7 devices connect to its wi-fi) thanks. Chris