I have purchased a mountain bike a couple of months ago and when i brake they make a really loud screeching noise , i thought it might have been the brakes pads so i bought new ones but its still happening please help

I think the pads and brake surface might have a layer that has to be broken in. Give it some time and it might go away or you can try sanding a layer off the surface.
Also, what pads are you using?

the pads are called jagwire , my local bike shop said it might have been something on the wheel and to try soapy water and i did try it but no luck

The dreaded brake squeal!

Ok this is what you do. Toeing brakes is a last resort.

You want to first add a new surface to your brake pads. Grab a file and file them down.
Brillo pad the surface of the rim.
Make sure the pads are hitting square (at the exact same place on both sides of the rim)

If this doesn't work you can toe your brake pads, but I would rather replace my pads then toe them since you lose allot of power when you toe pads.

Haha soapy water! Soapy water will make the pads stop squealing for a day until the rim is dry. Might as well add some lube to your rims while your at it.

What type of brakes, rim or disc, makes a big difference to the answers