1: Canon EF 50mm f/1.8 Mk II
The Canon EF 50mm f/1.8 Mk II is without a doubt the best value for money lens you can buy. Not only will you get a lot of shooting out of it, the quality is almost as good as the f/1.4 version. While the build quality will not earn much praises, it is definitely good enough to last you long enough that when it does eventually gets damaged, you will have no hesitation getting a replacement. This is definitely the best lens to start with in the world of Canon primes and it's very rare that you'll see a dissatisfied owner. You may move to better options but you'll be hard pressed to find anything remotely close in terms of value for money.
2: Canon EF-S 55-250mm F4-5.6 IS Lens
The Canon EF-S 55-250mm F4-5.6 IS Lens is a very economical option for those who need to shoot telephoto. With a 35mm equivalent of focal range of 88-400mm, this lens will give you plenty to play with as you understand your 550D better. It comes with image stabilisation so you comfortably shoot handheld under decent light conditions. Are there better telephoto options you can consider, sure... but unless you are determined to do a lot of telephoto shooting, this lens will allow you to invest your money into other areas of your gear. Either that, or save it towards getting the 70-200 f/2.8L IS II. Once you've mastered the 55-250, you will be ready to get your money's worth out of this much more expensive upgrade.
3: Transcend 16GB Class 6 SDHC Memory Card
A memory card is without a doubt a very important component in your overall workflow. Some prefer to have multiple cards (which I wholeheartedly agree with) while others will always go for the largest sizes possible so they won't be interrupted with a full card. I currently shoot with multiple Transcend 16GB Class 6 SDHC memory cards as the price for these are much more reasonable than their 32GB counterparts and at the same time, give you enough room for a large mix of JPEGs, RAWs and HD movie files. If your budget allows, I would say go for the Sandisk 16GB SDHC Extreme Class 10, but considering that its price is almost double that of the Transcend, you'll just have to decide if the additional performance is worth the cost. My Transcend cards have been good to me so far but then I tend to shoot in fair conditions and treat my cards very carefully on my social shoots. However, with work related shooting, I always go with the Sandisk for the extra peace of mind.