Why Orks then? First and most importantly of all they are a simple army to play. As an Ork General you really only need to know two things to give your opponent a hard time: you need to be in close combat as soon as possible, and you need weight of numbers. This knowledge will help you to build an effective list and execute on the table, for a beginner I would recommend a mechanised horde-style list; Battlewagons and Boyz! Keeping things simple is very important when you’re learning a game.
Something else which comes as a bonus of having weight of numbers on your side is that you will be rolling a lot of dice. This is important because 40k boils down to being a game of chance and if you think you can beat the odds, you’re kidding yourself. However, knowing those odds is a powerful tool in being effective on the table and throwing a lot of dice means that you are more likely to hit the statistical average over the course of a game. You may ask, "what's so good about being average?", well I think it means that you are less likely to lose games due to "bad dice". Sure the chance of making a bad roll at a key point in the game can never be written off no matter how many dice you're rolling but playing with you eyes open both literally and metaphorically (and throwing a lot of dice) makes executing a strategy a lot safer and easier.
That pretty much covers gameplay reasons for starting out with Orks: you can win with a simple strategy and you're less likely to be let down by "bad dice", however these are not the only reasons why Orks are great for the beginner. There is the extensive range of cool looking models including those which are part of the bargain that is "Assault on Black Reach", the fact that Ork Boyz are the real core of the army and that they can be purchased very reasonably on eBay makes this an affordable army to collect and almost certainly the cheapest horde army to get into.
Typically it's very easy to convert Ork models which further brings the cost of the army down. Practicing basic conversion techniques on Orks is a great way to learn as it's difficult to make an Ork model look bad because Orks are supposed to look a bit shabby and slip-shod. To help you in this endeavor GW have kindly put tons of bits and gubbins on their sprues which you can use to personalise your mob!I don't want this to go on too long so just one last reason why you should pick Orks as a beginner, it's an army that will grow with you as your skill level improves. While the army is simple to play it has a depth of strategy and a range of different lists you can build which give the army a great deal of variety and will keep it interesting way down the line. Forgeworld also produce a vast array of crazy Ork and Grot contraptions so if you decide to go down the Apocalypse route you're well catered for there as well. Frankly Orks are just a lot of fun to play, so I say to you, unburden yourself of the shackles of Power Armour and join the Waaagghh!