I am going to apply for visa to go Germany, but I have an idea to go Belgium too. Is it possible to enter Belgium if I have visa for Germany.
Germany and Belgium are both members of the Schengen Area - a collection of European countries between which there are no routine border controls and inspections. Because of this, they also have a common visa system for visitors. If you receive a visa that lets you in to the Schengen Area, you can visit any country within the Schengen area (normally - there are a few exceptions regarding visitors from disputed regions).
So, is there a central "Schengen" visa office that you must apply to? Unfortunatley not, instead you must apply to the country that is the main focus of your visit. In your case this would be Germany. You should find the details on how to apply on the website for the German embassy in your country. The German Federal Government provides a list of all their embassy websites.
Note that for some nationalities and individuals, getting a Schengen visa is not trivial. You should be careful (and truthful) in your application, and provide all the information required, and carefully read the (offical) guideance. You should be cautious when dealing with agents who offer to handle your application for you - this means that offical replies will not go to you, and you are not in control of what is submitted. Scams are known where the agent does not submit the application at all, merely keeps the fee themselves and tells you that you were rejected.
It will depend entirely on the type of visa you get from the German embassy. If it is restricted to Germany, then you are not authorized to visit other countries.
If the visa is valid for all member states, then you can visit Belgium as well.
There are no border controls between Schengen states - I recently traveled from the Netherlands - Switzerland - Spain and back without once being asked to show my visa or passport (well, once, but it was to check the id page and not the visa section).
However, this was before the recent attacks in France, and keeping in mind some of the security forces also had operations in Belgium, I would not be surprised if there are more vigilant controls at Belgium.
you will need to take note of the port of entry rule. you have to be sure that you are going to germany first, before you can apply for a visa at the german embassy. It is not advisable that you get you visa and then decide to go to belgium first.
And yes, you will be able to go to Belgium or any other schengen country if there is no restriction on your visa.