Your first contract

Create src/Counter.sol

// SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.13;

contract Counter {
    uint count;

    constructor(uint _count) {
        count = _count;
    }

    function increment() public {
        count += 1;
    }

    function decrement() public {
        count -= 1;
    }

    function getCount() public view returns (uint) {
        return count;
    }

}

Write the test for it (test/Counter.t.sol)

// SPDX-License-Identifier: Unlicense
pragma solidity ^0.8.13;

import "forge-std/Test.sol";

import "src/Counter.sol";

contract TestCounter is Test {
    Counter c;

    function setUp() public {
        c = new Counter(0);
    }

    function testValue() public {
        assertEq(c.getCount(), uint(0), "ok");
    }

    function testFailDecrement() public {
        c.decrement();
    }
}