I've Checked everywhere, and i found no guides at all with items, so i'll just start one.
If no one have objections and gives a proof there have been or IS a thread with lists of items and how's it used...

Let's just get started :

Green Shells
The main item to use, as it can be both fired backwards and forward to provide a wider range of attacking, but can be dragged along the kart/bike for protection from other projectiles, the green shell can also ricochet so you can be tactical by hitting the wall in front of you and avoid it the moment it gets by you, to trick the one behind you

Triple Green Shells
Works exactly as the Green shell, but it's surrounding the vehicle by three spinning greens, and the fires and ricochet as usual, but you'll have an ammunition of three, this is best for protecting against reds and bananas.

Red Shells
Works exactly as the green shell, but it will home in to the opponent in front of you, but it will lose it's homing trace when fired backwards, would be to cheap if it homes in to people that's behind you, red shells do not ricochet, they'll fall apart on first impact with anything, even bananas.

Triple Red Shells
Works exactly as the red shells but more effectively, as they have the same spinning feature as the triple green, but they still home in, and if you fire them in the right pattern, you can hit the one in front of you, three times, and earn even more than 10 seconds!

Blue "Spiney" Shell
The blue shell is the ultimate punisher for the current leader, as it zips by to the top, while everyone else will watch helplessly when the leader gets bombed into the air, and maybe lose to many places, also if you're targeted by a blue, you can use a Mushroom to bypass the explosion or just brake and take down everyone else!

Banana Peels
This slippery suckers can cause much trouble on ramps and various of other places wich decreases the opponents speed drastically by falling off course with possibly a very mighty item indeed, so be careful, also a banana can take down green and red shells as well for protection but not anything else, so place them wisely and don't run over your own bananas!

Triple Banana Peels
This triple bananas works exactly as the original banana, except they'll be lined up behind you in a row, they are basically best protection from greens and incoming reds, and it's better to carry them, rather to drop them.

Mushrooms
Mushroom is great when you're falling behind to much, as you can gross over gaps or offroad sections by using this small little friend, but if they are used over a banana or fake item box, you're in trouble, the mushrooms remains on the ground when dropped by force, and will instantly be used as a boost for the one who picks it up!

Triple Mushrooms
Works exactly as mushrooms except you can use three, better for going on long routes where more mushroom is needed, very great to save until later as well if you're doing great!

Golden Mushroom
The Golden mushroom gives you an infinite amout of mushroom for a short time, very good to take a route where items and weapons CAN'T hit you properly, but be careful, once used, and the timer will tick on and eventually run out on it's own!

Fake Item Box
The Fake item box is the secondary mining object and it's a little bit more effective than the banana as the victim will be blown up as well as dropping on holding items as well, plus this object cannot be destroyed by red or green shells, the only thing that works is bombs or blue shells, the only way you can see it's a dud, it's the upside down question mark inside it, and it's glowing red!

Bob-Ombs
The bob-ombs is a highly dangerous weapon in the right hand of an item user, as it'll explode with a radius enough to take down 4 - 6 at the same time, if not better, it can be tossed forward to ensure the player there will be blown up, or it can be tossed behind to surprise the one behind with a large bang, and for you, a good laugh, but be careful, if you carry a bob-omb behind you, someone might hit you with an item and blow up you as well as the ones who's next to you!
This item is hard to use, so you'll have to train to use this most effectively!

Mega Mushroom
This thing is really great, as you use it, you'll get a offroad and grip bonus to it, plus the size of an elephant to squash your opponents, but be careful though, as when time runs out, you'll be vulnerable again, you are invincible against everything, except for Stars and lightning (Explained later), but this thing should be used for protection most, and to surprise boosters who tries to knock you off course... har har har...!!

Starman
This is a perfect item for recovery, as it works as a better version of the mega mushroom, it'll add speed and gripping ability to your vehicle, and the ability of hitting courses obstacles as well as items on the ground, to smash, it's a good thing to use this when you feel you can't survive much of the course as it's swarmed by items, the stars have no flaws, except speeding of course! Nothing can hit you.

Lightning Cloud
This is not an item, is a punishment, it'll increase the speed and the gripping ability to the weilder, and an offroad ability, but it'll shrink you if you're not careful, the best way of using this is to hit an opponent right before it'll shrink the weilder, as that person is now really too slow to catch up :)

Blooper
The Blooper is not actually very useful, it'll only speed down the computer ai, but against human players, their screen inked a lot, and makes it harder to see anything, good to use when there is a lot of items laying around, but some are really good playing with screen filled with ink, so it's not very useful...

POW Block
Pow block is good to use when people are speeding through the course and you're falling behind, cause it'll make the opponent spin out and drop ALL items, except those with MEGA mushroom, Star and Bullet Bill, but it'll also only hit you when you're on the ground, so stay airborne, you can also avoid it by wheelie the right time (Even in karts), but you'll still loose the items.

Lightning Bolt
This is great when you're far too behind everyone, as it'll shrink EVERYONE on the course, and turn them into smaller beings, they'll also spin out and loose all their items as well, but be careful, those with megas will only turn back to normal size again, and star and bullet billers won't react with this, unfortunetely, it can be useful when the opponents are over a gap or three. (Cannot be found until 30 seconds after start)

Bullet Bill
An extremly good recovery item, as it will blast through everything on it's way and is mostly flawless, but be careful on how you use it, cause it might throw you off the course when the timer of it's use goes out, it can also blast away people using Mega Mushrooms and bump off people with stars as well, but using this near the goal on a good position is also good, plus you can avoid Shrinking effects while it's used!
While it's used you CAN'T pick up other items!

Thanks to BBA322 for providing the pictures

Flammie's ADVANCED Item Guide!
Advanced Item Guide here!
Flammie's MASTER Item Guide!
Master Guide here!