To use an EVM (Ethereum Virtual Machine) and run some Contracts on a non production purpose Ethereum Blockchain on-line called Remix.

If you want, you can access to Remix's GitHub repository here and clone it.

When you use Remix, once you have runned your contract, you can trigger his functions by pushing buttons in down-right side of the GUI, Blue ones are for Getter functions, and Red ones are for Setter functions.

By using "public" for a variable, is directly making a getter function on it.

In a getter function, there is no need to mine a Block but, in a setter case, you have to mine a Block because you are changing the contract's Data.

If you are stucked with No visibility specified explicitly for function's error, you have to put public statement to all the functions in your contract, and you will solve the visibility issue.