In this game you are playing the role of the Mario. You are going through the Mushroom Kingdom, surviving the forces of the antagonist Bowser, and saving Princess Toadstool. Mario should reach the flag pole at the end of each level to win the game.
There is a lot of coins scattered around for Mario to collect and special bricks marked with a question mark, which may reveal more coins or a special item when Mario hits them. Other bricks may also contain some coins or rare items so if you have enough time, try to hit them as well. If Mario 
eats a Super Mushroom 
, he becomes a Super Mario 
— he grows to double his size and gains the ability to break bricks above him. If he gets hit in this mode he will turn back to regular Mario instead of dying. If Mario eats a Fire Flower 
, he becomes a Fire Mario 
— in this form, Mario gains the ability to throw bouncing fireballs from his hands. If Mario touches a bouncing Super Star (a rare power-up), he turns into Star Mario. In this form Mario is invulnerable to nearly everything and he can kill enemies with a simple touch. This is a temporary form so use your newly obtained powers quickly!
You can read more about the gameplay of one of the best games here.
Mario can attack enemies by jumping on top of them, though different enemies react to this in various ways. For example, Goombas flatten and are defeated, while Koopa Troopas temporarily retreat into their shells, allowing Mario to push them to defeat other enemies. However, these enemies might also bounce back and attack Mario, potentially injuring him or even causing his death. Mario can also defeat enemies above him by jumping and hitting them while they stand on bricks.
The game consists of eight worlds, each containing four sub-levels known as "stages." The final stage of each world takes place in a castle, where Mario faces off against Bowser on a drawbridge. [Spoiler Alert] The first seven Bowser encounters are actually "fake Bowsers," disguised minions posing as the villain, while the real Bowser appears in the eighth world. To defeat them, Mario needs to jump over them and reach the axe at the end of the bridge.
The game also features underwater levels, each with different enemies. Additionally, there are bonus and secret areas. Most secret areas contain more coins for Mario to collect, but some secret areas feature "warp pipes," allowing Mario to skip ahead to later worlds without completing the intervening levels.