This series of pages on Mathematics for Students has numerous useful links for students to check their calculations.
The ultimate Calculator has to be WolframAlpha which as you can see has a page of its own with several slideshows to help you learn how to use it.
For drawing and exploring graphs, use the outstanding Desmos Graphing Calculator.
For some other useful calculators and tools and accompanying notes see the following pages……….
The pages in the series: