After started, when I release the brake and squealing sound is heard.

A "Squaling" sound? Do you mean Squealing? Squealing from a stop to moving suggests an issue with the brakes. More information please.
When were the brakes last serviced? Some brake pads are fitted with a layer that causes a squealing noise when they are at their lowest safe thickness.
Is it one long, continuous noise or a short repeating sound. A long noise could be a sticky or seized brake calliper and a repeating noise could be a warped brake rotor.
Do you get any other noises? Such as when your braking or releasing the Park/Emergency brake?
Does it do it when the vehicle is cold like first thing in the morning or after you have driven it for a while? Cold could be a fluid issue and a warm could be something metal rubbing after it's warmed up and the metal has expanded.
Use the comment button below and I'll see if I can help you more.

You need to have the brakes inspected. This noise is the brake pad wear warning tab hitting the brake rotor.;

For squeaking problem,the help links are provided below.Click the link below to go through the procedure :----squeaking under hood? http://repairhelpcenter.blogspot.com/2012/01/clutch-squeaking-under-hood.html
----------Squeak and grinding sound is heard when brakes are applied? http://technoanswers.blogspot.in/2012/02/squeak-and-grinding-sound-is-heard-when.html
-------------Clutch squeaking? http://repairhelpcenter.blogspot.com/2012/01/clutch-squeaking-under-hood.html
---------Squealing at start up on car? http://technoanswers.blogspot.com/2012/01/squealing-at-start-up-on-car.html
-----------This will help.Thanks.

yes sounds like you need new brakes thats the brake wear indicator squeeling need new brakes

most likely

This sounds like a wear indicator( a metal tab on the brake pad) is doing its job, warning you that the brake pads need replacement soon. you can bend the wear indicators away from the rotor so they won't make contact. This will stop the annoying noise. Replace the pads and check the brakes soon. Brake pads should always be changed on both wheels at thge same time, so change the other front pads also.

I agree it is the brake pads and rotors. If it has been doing it for 6 months then it is pretty likely you need to replace the rotors. Had you replaced the brake pads soon after you started hearing the squeal, you might have gotten by just replacing the pads and not the rotor.

You need to tighten/replace your alternator belt as it is either worn or loose.

Sounds like your brake booster but can't be sure until I know what kind of truck or car you have....

Easiest way first.....if the belt is squealing (at idle), spray it with belt dressing or WD-40.....if it stops....may be the belt or the bearings in the alternator....if it continues only with driving.....brake wear indicator letting you know...brake job due!

Please let me know if this helps!!!