Try to play with items on from now until the tourney, so u get used to using them a bit. They can add a tricky extra element to the game that will hurt you if you arent good with em and the other players are. You never know, there are some decent smash players that never made it to smashboards and play with more obscure rules(items, 3-stock, etc.), but most likely they are n00bs and you will pwn them with your 1337 skills..
Oh, and is it all items on? Or are they turning off hearts, tomatoes, or stars or anything?
Anyway here are a few tips for using items:
1) often times throwing the item is the best choice, but not always, especially if you are good at being a nuisance with said item. People play you differently when you have an item in your hand, usually because they are scared of getting it thrown at them. <know this and crush them!
2) like asianaussie said, when using items like the beam sword and bat, use tap A and use tilts, the smash attacks are powerful but impractical because of how slow they are(often times your characters normal side smash attack is much quicker and more effective).
3) keep them on their toes by sometimes using useful items, and sometimes throwing them.
4) you can still use your Nair while holding onto item, and maybe your b moves, (i dunno about links bombs, or other ariels(i am a real man(i turn items off))but it might be good for you to test out).
When a character is off the stage its easy to gimp them with a thrown item, either directly at them, or while edgeguarding throw straight up while standing at the edge and when it falls it will add an extra obstacle for the person trying to recover.
All jokes aside, hope this helps a bit, and good luck.