Try to keep your cards at 60, so you'll have a better chance of drawing your better cards.
If you have many cards in your deck that cost alot, then I suggest placing about 20 lands in your deck. If not, then about 16-17 lands.
Try to keep only 2 or less colors in your deck; you might get the wrong lands at the wrong time, and would have to put extra of each land in.
You don't want the most of your cards to cost too much; You want most of your cards to have a low cost.
Green: Just make sure you get land boost cards out, like certain cards have the ability 'When this card comes into play, put a basic land card from your hand into play...' Those are good to have. Also, get powerful air blockers, like that spider guy, who can block air as well.
Red: Red is excellent on destroyed your enemy quickly. For example:
Lava Axes (5 Damage)
Book Burnings (6 Damage)
Volcanic Hammers (3 Damage)
Scorching Missle (4 Damage)
Shock (2 Damage)
And theres another card that deals damage equal to the number of mountains in play. If you have enough mana, and the right cards, this can be an easy win.
Black: One good idea is having an all Zombie deck. There are alot of good zombie cards, like;
Bone Knitter (Regenerate Target Zombie)
Organ Grinder (Deals 3 Damage to player by removing three cards from graveyard)
And much more. Also, the legendary Pain Bringer:
(Remove target card from your graveyard from the game: Target creature gets -1/-1 until end of turn). Not to mention you can play this as many times as you want. So it might be good to put some weak cards on the battlefield and try to get them killed so you can easily remove them from the game.
Blue: Good for air, especially famous for Counter Spells. This deck requires alot of mana, but its mostly worth it. I suggest an all flying deck, because thats just what they are good for. Also, stick your deck full of Counter Spells. A blue deck would also go good with a Black deck mixed in.
White: An excellent color. Mostly used for protection spells and healing spells, but powerful at the same time.
Three words: Circle of Protection.
There is a circle of protection for every color, what this does is prevents all damage from that one color from happening to you, and from your creatures. This could very well change the outcome of the game.