How to use second access point as repeater (by 2 users)

I had an access point (D-Link Type) in my office for 5 PCs in ground floor and connected ADSL modem to the access point as router.
but when I had to add another PCs in my office in first floor , the signal was very low and I had to buy another D-Link access point.
But when I turn it on, there is some problem in my wireless network and some of my PCs is out of range of my network
I asked somebody and they told me you should use second access point as repeater. pls help me how should I config socond access point as repeater.