Using 2 Memory stick won’t work on my PC

My computer is an old version. And so it's very slow. But my motherboard's have two slot for ram. And my Ram is DDR1.
But it is marketed out. I tell my friend if you get any Ram of DDR1 then you tell me I bought it. And now I get ram. Now my problem is two Ram do not support.
When I out one ram then the computer is running but two Ram cannot run my computer.
So what can I do about this problem?
