So, you're going to be trying out Eldar for the first time in a competitive environment? Sadly, I'd suggest you not get your hopes up. Eldar have a very tough learning curve, and even when you get them down, they struggle against top-tier armies. Try playing some practice games before the event, even if it's just on Vassal.
In terms of what you currently have:
Farseer: Awesome choice, always include at least 1. Doom, Guide, Fortune are the only good powers. Take 1 of them, unless you take Spirit Stones, then take 2. Always take RofWarding on 1 Farseer. RofWitnessing if you
need your abilites to work (ie to keep Fortune on a Farseer+Warlock squad).
Warlocks: Also a solid unit. They're too expensive to run in a massed squad at 1000pts (
IMO), but at 1500, they can work well. It's best if they're on Jetbikes, but they can work in a
WS too. If you do take them as a unit, don't skimp out on their number. The more, the better. Take a few Spears, but don't overdo it. Give 1 Enhance, 1 Embolden, and a few Destructor. Some should be run with no powers to keep points down.
Avatar: Works great in a foot-slogging list. Leave him at home for a mech list.
Jain Zar, Karahdras, or any other
PL: Don't take them. They aren't worth NEARLY as much as they cost. Use them as cool models on your shelf, nothing more.
Scorpions: I like them, but they aren't particularly good. To be competitive, leave them out. If you must take them, fill out the squad as much as you can, and take a Biting Blade over the Scorpion's Claw.
Dragons: Excellent! Take a single squad of 5, no exarch in a
WS with a
TL-Cannon, never upgrade this! Anything more is simply a waste of points. (Although Stones on the
WS work) They're a suicide squad: flying in, taking out any tank you want nearly 100% of the time, and dying. At higher pt games, you can take 2 of these squads, but 1 should be enough at 1500.
Banshees: These are like Scorpions. Keep them at home. If you must, take them in a fairly full squad (8 or 9) with Exarch+Executioner. War Shout can be taken if you have
nothing else to use points on, but it looks way better than it is.
Harlequins: A few people like them, but most of us really don't. Eldar simply don't have a good
CC squad that actually works well in game. If you really want them, don't bother with the Jester. You'll want them running or assaulting, he'll just slow them down.
Wave Serpent: A must for any competitive Eldar force. You'll need more than 2, sadly. For the Fire Dragon
WS, take a Cannon or Scatter Laser. For your troop or Farseer+Warlock
WS, take an
EML or Brightlance.
Dire Avengers: These and Storm Guardians are the only competitive troop options.
DA squads can either be: 10DA+Exarch w/ dual-catapult+Bladestorm, or 5DA no upgrade. The 5
DA work to either to just make a
WS or Falcon scoring, they usually will never leave their transport or shoot a single time. Whatever option you take, they need a transport.
Rangers: They are 'meh'. A single squad upgraded to pathfinders can work just to sit in an objective that's in cover and take a few pot shots. Don't overdo them, and I recommend leaving them out entirely.
Defender Guardians: Sadly, not at all competitive

They just don't have the range or survivability to do much of anything. A single heavy weapon with BS3 isn't enough to make up for how quickly they'll get wiped off the board by either shooting or
CC.
Storm Guardians: Shockingly effective. They work about as well as
DA squads (some say better, some say worse, I say 1 squad of each!). Take them with the following loadout: 10 Storm Guardians + 2 Flamers + Warlock with Destructor +
WS. They are
NOT a
CC squad, they are a triple-flamer squad that (preferably with doom) burns things to death.
Jetbikes: I'm not a fan, but some people are. Run them in a group of 3 with a cannon upgrade and accompanied by a warlock on jetbike with embolden.
Shining Spears: Nope, too expensive.
Warp Spiders: Best option for
FA choice. Take them if you can, but they aren't necessary.
Hawks: Nope, totally worthless. But, they look cool!
Viper: I'd stay away from them, they just don't work at well as War Walkers. In high enough Pt games, you can squeeze them in, but not at 1500.
D-Cannon: All the support weapons are rubbish. Don't bother.
Reapers: They seem cool, and can really punish
MeQ in the open. But, you won't see
MeQ in the open from a competitive player. Don't take them.
Wraithlord: Looks good on paper, but can't keep up in a mech list.
War Walkers: Excellent unit! I like to take a squad of 3, each with Scatter Lasers, but some people prefer Cannons to keep their cost down. If your Farseer is near them, toss them a guide and watch them tear things apart.
Falcon: Works well. Don't upgrade to a cannon. Try to put a squad of 5
DA inside so you can use it to score at the last moment. Anything else in transport is a waste. For a gun: go with
EML or Brightlance.
Fire Prism: The best
HS choice. Take 2 of them, and either give both or neither Holo Field.