I've been collecting Magic for several years now, on and off. Recently, I've introduced a few friends to the game (from my new 40K gaming group) and have discovered a thriving magic community at school. As I write I have just got back from the wargames club where I lost two matches and conceeded one. Having lost so many times, I feel I have the knowledge as to what does and does not make a good deck- It's just that I can't do it myself!
Choosing your colours
Choosing your colours is vitally important. Too many will mean you never get the mana- too few and a couple of Hosers will crush your deck. So, I would reccommend 3 colours if you're new or in a sealed environment, otherwise I'd say two is right as it is hard to balance 3 colours well and still ahve a functional deck- though you could emphasise two and splash a third. Here then is my guide to colours.
MONO-BLUE
A mono blue deck will have great scope for harrassment- you could go for a counter deck where pretty much every spell they cast fizzles, or atttempt creature advantage by constantly gaining control of or bouncing back to their hand your opponent's creatures before hitting with your own. If you're looking for a good starting place, consider investing in the Dominator preconstructed deck- it has a strong basis of counter and control, whilst unusually for a blue deck having a lot of powerful creatures or those with evasive abilities.
What blue does tend to lack is direct damage, apart from the occasional TIM barrage. For this reason, you want creatures with shadow, landwalk or flying. Forget banding- your opponent will generally have many more creatures than you and so little will get through. Beware that many blue cards have islandhome, so unless you're playing against a blue deck, bring along a couple of phantasml terrains. Until then, Use big creatures as powerful blockers. Propaganda is a very useful for a blue deck short on creatures, as the other player will seriously struggle to bring cards into play and attack on the same turn. Two would be a very sound, if somewhat cheesy, investment- each attacking creature will then require 4 mana!
There are two colours to particularly fear when playing blue. First, a red deck is fast. Cheap, high damage effects are available and you won't counter them all- so expect to take heavy losses until you manage to gain control later in the game with more islands in play. As blue lacks much in the way of life-gaining ability, you'll want loads of various counterspells- manaleaks and spellblasts will do fine against lightning bolts early in the game until you can use the more powerful counterspells for fireballs and the like. Find Ertai, his counter-a-turn for 4 mana is a bargain, though he is fragile and as a legend there can only be one. Also look out for black decks- the numerous discard effects will ruin your ability to counter and control.