Its probably already been said, but there's a tutorial at the beginning if you wait a few seconds.
But for how you play (controls described for where originally mapped, can be changed in settings)
Circle Pad left/right=move left right
Circle Pad down=crouch/drop down from platform (some characetrs can move while crouching)
Circle Pad up=jump (also x and y buttons will do this as well)
on ground
A=normal attack
light tap of Circle Pad+A=tilt attack (3 variations: side, up, and down)
hard press of Circle Pad+A=smash attack (again, 3 variations: side, up, and down, charge for higher power or just release instantly... will automatically go off after about a second of charging though)
B=standard special attack
Control Stick+B=tilt special attacks (once more, 3 variations: side, up, and down.... up special is also typically though not always your character's main method of recovery if jumping doesn't work well enough)
L=grab
while grabbing any enemy
L/A=grab attack
Circle Pad=throw (side, up and down variations)
R=shield (it drains by being attacked as well as just keeping it out, and if it breaks you're temporarily stunned)
while shielding
Circle Pad down=dodge
Circle Pad left/right=roll in direction (you will end up facing the opposite direction you rolled, so roll left to face right and vice-versa)
in the air
A=aerial attack
Circle Pad+A=tilt aerial attack (this one has 4 variations: up, down, and a front and back side depending on which direction you're facing while in the air. so if you're facing left, left+A will be front aerial, right+A will be back aerial)
R=aerial dodge
see special B moves listed in grounded section above, they generally all work in the air as well
also some characters like Donkey Kong can also grab enemies while in midair
I think this is all the basic control stuff. Probably even more than you needed actually lel.
Goal is to get your opponent flying off the screen via having them take enough damage pretty much, though stamina matches also have the added goal of making you want to get your opponent down to 0% health. However if you knock them completely out of the stage, it will count as knocking them down to 0% health even if they're at high numbers still. Also coin matches have you collect the most coins, but I don't think I've seen that in 3DS version? Or maybe I'm forgetting.
Anything else, just ask. It's not too terribly hard a game to understand or even play once you get used to its controls. Higher difficulties could be troublesome, but unless you're trying to 100% everything the game is pretty generous with letting you stay on lower difficulties with just a few characters you get good at.
- - - Post Merge - - -
oh and A also picks up items and then uses/throws them if possible depending on the item
example
pick up assist trophy=use automatically
pick up raygun=A will fire a shot and will continue each tim you press A until you run out of ammo in wgich case it will be thrown
pick up Pokeball=A will throw it releasing the Pokemon inside
Smash balls (glowing things with Smash Bros. logo that fly around) will be destroyed if damaged enough, and the character who destroyed it gets to use their Final Smash attack. A super powerful attack that will generally if it connects KO the opponent, or failing that, make them take a lot of damage at least. Which you'll know if a character can use it, because the screen will darken and that specific character will have the same glow around them that the Smash Ball did. Final Smashes can be dodged however, though the degree of difficulty varies with which character it is. Like for example Link's (which requires you to line your opponent up with you) would be easier to dodge than Pikachu's (which has him flying all over the screen controlled by whoever is playing him). Also, its entirely possible to knock Smash Balls out of opponents by being attacked, in which case they or someone else can then try and attack the Smash Ball again to take it for themselves. Lastly, Smash Balls will vanish after a time if nobody destroys it.
B=use Final Smash (grounded or aerial, though can't be tilting the Circle Pad)
