I suppose parsing strings was super vague, what I meant is your calculator should take as input a single string and process it.
For example parsing "1 + 4 - 2". While this can become arguably complicated it can start off quite basic and grow to a great degree depending on the complexity you'd like to achieve, for example implementing brackets.

Also I'll give you a hint, using a stack based implementation will probably be the best way to go with this problem. Familiarise yourself with queues, stacks, and other common data structures and have a think about that.