I want to calculate tolerance for my design product but don't understand where to start and do?
Tolerance is most often a function of the manufacturing process and part size, so first you need to decide how each part in your design is to be manufactured. For example, small screw machined parts will inherently have much smaller tolerances than large, folded sheet metal parts.
Conversations with potential suppliers of the parts are always a great place to start because they know better than anybody what tolerances they will be able to hold.
Once you have an idea of what tolerances you can expect from the processes and parts, you then need to consider how those tolerances will play in the design. This is usually called a 'tolerance stack analysis' and involves calculating and comparing the worst case conditions, for example, if a dimension comes in large, will it still fit when it's mating part dimension comes in small? When the answer is "no", then you have to figure out what can be done to the design to get the answer to be "yes".
Sometimes it's simple, like making a hole a slot instead, while other times you find out there's no way to get it to work and you have to re-think the design.
Generally speaking, it's best to have a pretty good idea of the tolerance capabilities of the anticipated parts and processes before getting too far down the design path.