I'm assuming you mean sales tax in the U.S.
The funny thing is, sales tax varies between 1-10% of the dolar in the different states of the U.S.
The government gets their money somehow though! If it isn't through sales tax it's through property tax, income tax, etc.
Here's a chart showing how it differs from state to state, but it all adds up to about the same amount!

To find the sales tax for your state, (again this is assuming you live in the U.S., you could live outside the U.S.) I would just google "[state] sales tax."
Once you have this percent, change it to a decimal. If you got 5%, the decimal would be .05, if you had 7%, the decimal would be .07, etc.
Now multiply this percent by however much the cost of a certain product is, and it will give you the tax on that product!