I just bought a 24 in Full HD monitor but when I go to play a game like minecraft it is smaller.How do I make it fullscreen?

You can try changing the screen resolution

Lol. Welcome to the LIMITATIONS of playing Atari 2600-caliber games!

Depends on what is full resolution in pixels of the "24 in Full HD monitor" of some unknown brand, model number, and what resolution the "minecraft" game was designed for. With no information, no accurate guessing for you.

I usually play minecraft in a window, but maximize the window on my 24" 1080p resolution screen. If your resolution is 1080 and you are setting minecraft to full screen and it does not fill the screen, then you will have to lower your screen resolution to match that of the game.