I have a laptop and a music mixer software in my laptop and I wanted to connect it to the speakers of my house. Does anybody know how can I do it?

Your laptop should have a stereo output connector on its side or its back. That's usually a 1-volt peak-to-peak signal. It's usually a 1/8" stereo phono plug, if I remember properly (too lazy to look). If you take that and connect it it into the auxiliary inputs (a pair of red and yellow RCA jacks) of the amplifier that drives the house speakers, you will undoubtedly be successful.

You can also connect the stereo to the headphone output, if you don't have the 1 volt peak-to-peak signal. It will then have to be amplified and it might be a bit noisy, but it will work.

Use your headphone jack as output to your speaker unless you actually have a line-out

use your headphone jack as output to your speaker unless you actually have a line-out

If no line out the get a 3.5mm to rca(female) then a long rca cable and go to a input on your stereo receiver.

When using your computer switch input to appropriate one your stereo.

So...
[laptop]-{3.5toRCA}{RCAcable}--------[reciever]

Hope I made sense