Ok..So lets see how to add different tax rates to different products in osCommerce. The whole idea is very simple. You have to have a tax class with the desired tax rate and then attach it to the product. Following is the step by step guide for your ease.

Step 1:

The very first thing we need to do is to create a new tax class. Because one tax class can contain only one tax rate so we have to create a new class whenever we want to add a tax rate which is not there in any tax class. So in short, if we want a new tax for a particular product we need a new class.

To add a new class login as administrator.
        Location/Taxes >> Tax Class
Add a new class here by clicking on the "new tax class" button.

Step 2:

Now we have a tax class but the tax rate is still to put. For that go to
        Location/Taxes >> Tax Rates
Click on "new tax rate" button and it'll ask for the details. Select tax class you've just created. And put the tax rate etc.

Step 3:

Now the final step. You have a new tax rate and now you have to attach it to a particular product. For that go to
        Catalog >> Categories/Products
Double Click on the category where the product is. Then select the product and click "Edit" button on the right.

Change the Tax class to the one which you've just created.

Thats it. Done.

In case you have not created the product yet but you will create it after adding the tax class then its even simple. Just change the tax class to whatever you wish.

Hope its helpful !