Deckbuilding 101

WELCOME TO DECKBUILDING 101
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Here I will try and pass on some tips and knowledge on how to build a good deck. I hope you can read this and learn something new, or use it to your advantage somehow.

I treat constructed much in the same way I treat drafts.. When building/tweaking a deck I look at;

Removal: Do I have enough removal cards to make sure my opponents don't kill me.
Can I deal with creatures?
What if they have a planeswalker, or an artifact?

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Win-cons: Do I have enough ways to achieve a victory? Do I have more than one way to win a game? A deck should always have some kind of backup plan.

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Bombs: Whether they are planeswalkers, creatures or some giant pain in the ass artifact, your deck needs some kind of bomb. Something for the deck to center around.
Bombs usually ARE your wincons, but do you have enough bombs to guarantee getting one in every game?

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Land: Without land, your deck is nothing, NOTHING. So here is a quick guide to how much land you need to make your deck work.
This isn't gospel, it's a rough guide, and its what I use.

Look at your deck, and figure out it's "curve". See where the most important mana point is in your deck.
Do all your important cards cost 3 mana? Do you have bombs at 6 mana?

Curve of 2: nothing in the deck really costs more than 2 mana.
You can run about 16-18 lands, you only need 2 lands, any more will be adverse to your strategy.

Curve of 3: nothing in your deck really costs more than 3 mana.
You can run as low as 18-20 land in this deck, but try to avoid lands that come in tapped as you obviously have a fast deck.

Curve of 4: 21-23 land. Hitting that fourth mana is obviously important and you need a substantial amount of land to get there.

Curve of 5: You want to be running about 23-24 lands now, 5 mana is difficult to hit and balance the amount of lands/threats you draw.
You should really be looking at a way to draw cards in this deck now.

Curve of 6: 24-26 lands: As above really, card draw or mana ramp (rampant growth, cultivate etc) become pretty important, otherwise you could be drawing land all game and not hit enough answers.

Curve of 7 plus: Wow, someones greedy. Decks like these can run anywhere from 24-28+ lands.. Usually with a lot of mana ramp cards to make sure you hit these "fatties" quickly.

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A balanced curve is pretty important, its no good having 20 massive 5-7mana cards if your opponent is going to kill you on turn 4.
So you need to have ways to protect yourself in the early game, small "chump" block creatures, removal, fog effects (no damage). Ways to make sure you survive.

Swings the other way too, if you have a really low curve, focus on being fast, really fast. Your creatures and spells are obviously not designed to take on big creatures and threats, so you have to think, can I beat my opponent before turn 4/5 regularly? If not, start tweaking that deck.

A good curve makes the use of every mana slot, so you are hopefully doing something every turn. A turn you're not doing anything is a turn where your opponent is getting an advantage.

IE my red deck. Turn one, play a mountain, pass the turn.
I didn't play anything, but I have many 1 mana instant removal spells that I will likely use during my opponents turn.
So whilst i'm not casting on my turn, I make use of that 1 mana, or atleast, I have the option to make use of it.

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Finally, a sideboard.

Not essential in casual magic, but if you turn up to Archmage it is a massive advantage.
What is a sideboard? It's 15 cards you keep in your deckbox, these cards can be brought in after your first game against every opponent.
At the end of the match, before you play against someone else, you must put the deck back to how it was when you began the tournament.

Why use a sideboard? If you play against someone game 1 who has creatures you simply couldn't answer, or horrid planeswalkers you couldn't defeat, then for game 2 you delve into your sideboard and fetch out some answers for these problems..
Basically, a sideboard fills in all those holes that the opponent may try to jump through to kill you.

What should you put in a sideboard? Artifact/enchantment removal is usually a good one. Specialised removal is also good.
Wrath effects: cards that deal damage to all creatures, or destroy all creatures.. These are good to bring in against those fast decks.
Protection spells: Stop them from killing your lovely creatures.
Lifegain: Fed up of that deck killing you too fast? Try gaining some life in game 2 ;P

If you have any questions, feel free to ask them.

The word "Removal" should be changed to "Answers". It dosnt have to be removal to deal with something. Counter spells, control magic effects and board controll all perform the roles you described with removal. Small knitpick i know, but importnant. Dont want people trying to find Doom Blades for their G/W deck when all they need is Glare of Subdual.

Also your land advice should be severearly ramped up. And you should do some calculations on actual mana curves. A curve of 4 means "The average mana cost of the deck is 4". Which may not sound like a lot but it is. An avage CMC of 4 is really rather high. An average tournament deck will have a curve of something like 2.7.

So:

Average CMC < 1.2:
16-18 Land. This is decks like affinity, or signal pest decks both rely heavily on free cards like memnite or Ornithopter.

Average CMC 1.2 - 2.2:
19-21 Land. This is decks like white weenie or decks with a lot of inherant mana generation like elves.

Average CMC 2.2 - 3.3:
22-24 Land. This is the average area and as such falls into the typical land amount (23). Decks that have a reasonable curve with a few high end cards not usually exceeding 6CMC. With maybe the odd few mana generators or card drawing.

Average CMC 3.4 - 4.5:
24-26 Land. Past here is gets a little more complicated. Typically decks that play upwards of 25 lands are not doing so because they need a lot of mana, but because they have either strong card draw engines(U/X Control) or the ability to cycles through a lot of land (knight of the reliquary) or they might even have colour fixing issues (Jund). But if your curve does happen to be be in this range then 25 lands may be what you need.

Average CMC > 4.5:
Dont go about 27 land is kind of a rule of thumb. If you need that much mana or fixing then run creatures that do it for you. If you cant do that then dont build the deck, it's going to be shit.