Whistle

You need a couple of good whistles to start and stop play.  I had a good old-fashioned pea whistle I bought a few years back when I started coaching.  I added a Fox 40 plastic pealess whistle to this for refereeing.  Both produce very distinct sounds.  The pea whistle is the classic rolling whistle sound while the Fox 40 is a single note that is pretty high pitched.  Why both?  If you're refereeing a youth game at a playground or park, it's likely that you will have other soccer fields adjacent to you.  If you notice that the players are stopping on your field because the ref on the adjacent field blows his/her whistle, you'll want to switch your whistle.   Wait until the ball is out of play to switch and let both teams know what you are doing.  They'll appreciate it.

Lanyards:  This is one of those personal preference issues.  Most of the catalogs sell three kinds of whistle lanyards:  neck, wrist, and finger.  My clinic instructor recommended that we not use neck lanyards because of the amount of running a referee does.  The whistle around the neck of a coach isn't so bad, but when you're sprinting down the pitch, your whistle is going to be bouncing this way and that on your chest.  This is going to make it more difficult to grab. 

OK, no problem, you say.  I'll just run with it in my mouth.  WRONG!   Don't keep the whistle in your mouth, because that's when you're going to get hit in the face with the ball, and you'll probably break some teeth.  If you're not sure between the wrist and the finger lanyards, buy one of each.  They cost around two bucks each.  Use the one you like better on a regular basis, and keep the other in your pocket as a backup.

Flags

The Center Referee (CR) usually provides the flags for club linesmen (usually parents or older brothers/sisters in youth leagues) or for Assistant Referees (AR).  A set of flags can run from $10 to $40.  The budget-priced flags will do just fine for getting started.  Once you start earning those big bucks from your refereeing fees, you can consider upgrading.

Red and Yellow Cards

Hopefully they'll stay in your pocket, but you need them.  Give some thought as to where in your uniform you're going to put each card.  I keep the yellow in my left shirt pocket and the red in the right pants pocket.  Keeping them separate decreases the possibility I'll pull out the wrong card accidentally.

Match Reports/Wallet

Most of the equipment suppliers sell a small plastic wallet that holds your red and yellow cards along with a small pad of game report forms.  The whole set should cost $4-$5.  OSI and Law Five sell laminated card sets that you can write on with an erasable pen, so you can keep track of the cards you issue during a match.  I picked up a set of these immediately after working a rainy, wet weekend where it was all but impossible to use paper and pen.  I still prefer the paper match report to writing on the cards (particularly when I'm an AR), but we have enough rainy days per year that I could easily justify the $8.95 for the write-on set.

Make sure you put your card/report wallet in your pocket even when you're an AR.   You should still record the match as if you are the CR when you're on the line.   This helps the CR to keep things straight, and gives you practice filling in the reports.  If a fight breaks out on the pitch, usually the CR and the closest AR will move in to break it up.  The trailing AR would then pull out the report and take the numbers of players involved.  If you're the senior AR, you also need your cards and wallet in case the CR is unable to continue the match and you have to take over on the pitch.

Watches

The CR is responsible for keeping the official time for a match.  To do this, you'll need a good stopwatch.  I have a multi-function wristwatch that has hands and a digital stopwatch.  I record the actual start/end times of the half from the hands and start the stopwatch when I blow the starting whistle.  I just leave the stopwatch run and mentally adjust for stoppage time.  (For a more detailed technique on match time management, see Giovanni Piazza's great piece on this subject from SOCREF-L.)

Some referees will operate two watches during a match.  They'll start and stop one and let the other run continuously.  That's way too complicated for me, because I know I'll forget to start the stop/start one on a re-start somewhere along in the half.   I have a around-the-neck stopwatch I used to use when I was coaching that I keep in my pocket as a backup in case something goes wrong with my wristwatch, though.

Handkerchief

This was a suggestion from the book The Soccer Referee's Manual (see the Books page for details on this great reference) that I liked.  You probably already keep a handkerchief in your pocket for your own use, but it's also a good idea to put an extra one in your pocket.  Nothing fancy, just a cheap white one.   That way you'll have something to pull out to apply to a bleeding player or referee in case of an emergency.

Coin

Put a quarter or half dollar in your pocket for the pre-game coin toss.

Pump

It's a good idea to pick up an inexpensive ball pump and a pack of adaptors for the pump and throw these in your bag.  I remember one U-8 match where the CR rejected four balls from the home team before coming to me (I was the visiting team coach) for a fully-inflated ball.  If the CR would have had two coaches equipped like the home team, this could have been a problem.  Keep a pump with you and you'll be able to take care of the situation yourself.  Don't rely on inexperienced dads for these things.

Water Bottle

Even if you stopped at a convenience store and picked up a quart of your favorite sports drink, you may want to bring a water bottle along with you.  Some leagues can work a referee almost all day on a Saturday, so you might want something you can easily re-fill.

Bag

You'll need a fairly decent sized bag to put all of your stuff in.  My clinic instructor made an excellent suggestion on bags--buy a black one.  Here in New Orleans, many of the leagues have distinctive colors--Lafreniere is red, NOSA blue, PAC purple, Carrollton green, etc., so a black bag doesn't associate you (even incorrectly) with one league or another.

Well, you should be just about ready to take the pitch now.  Good luck!  When you get back, try exploring some of the Referee Sites on the Internet we've organized for you.