I know that for some people, this post might seem utterly stupid and probably utterly inexperienced, since I'm not a very good player myself. However, there are a few rules you could follow if you want to build a good deck for Magic. Rule no. 1: Know what you want to do with your deck. This is an important rule. A lot of decks are lousy because players don't have a focus on what to do with their deck. Just mashing together a set of good cards do not make a good deck. Some decks may look like piles at first, like GW Glare and Eminent Domain, but in truth, these decks have a focus. If you have a focus, you know which cards to put in, and which cards to not put in. When you are tuning the deck, you'll know whether or not your deck is working out. You'll also know whether it is worth to splash certain cards in or not.
Rule no. 2: Good cards make good decks. In the world of TCGs, this rule stands so the people who worked at these cards can make money. Most good cards are at rare, but do not let this dishearten you, because commons and uncommons can be superior in some respects. In most cases, this actually means putting in Sakura-Tribe Elder (which is a creature) instead of Rampant Growth (which is a spell) or Seal of Fire (which is an omnipresent threat) instead of Shock (which you have to have mana all the time to use). If you don't have the card, use a substitute. Final Judgment can replace Wrath of God in many respects, and is occasionally a lot more favourable depending on your match. Sometimes, there are no substitutes, but you can always work around that. Casual decks will have ways to get around their lack of rares or chase cards. (I'll discuss this later.)
Rule no. 3: Have a solid mana base. The mana base for a Magic deck is the backbone of the deck. A weak mana base will wreck the deck. Evidence of this is the high prices of the new dual lands. With a weak mana base, your Wrath of God which you worked so hard to acquire is nothing without the 2WW you need to play it with. Eminent Domain is a good example of an odd looking mana base that is solid. By using lands together with the Signets, we can see the mana base solve problems. In Kamigawa, the duo of Sakura-Tribe Elder and Kodama's Reach allowed previously impossible decks all on their own by helping decks fix their mana. If you have difficulties calculating the ratios, try counting every single mana symbol of each color and comparing them to get your ratios. If you are using mana fixers and mana accelerators, be sure to take them into account. It might take heavy playtesting to get the right numbers to optimise your deck!
Rule no. 4: Good decks play well against themselves. You should not only playtest it yourself, you should playtest against it to see what it can deal with. You should also playtest it against itself. If your deck can give itself a good run for its money, you have a good deck. If it depends too much on chance, that means your deck still needs a lot of work.
Rule no. 5: Great decks play well against all other decks. Occasionally, you may come into a completely metagame deck like Fungus Fires, RW Control and so on. While these may be good decks, they most certainly aren't great. Great decks can handle practically any matchup, and their worst matchup should be 40/60 or 50/50. If it is any worse than that, try again.
Rule no. 6: Netdecking isn't as good as you think. A lot of players netdeck. What a lot of them don't realise is that netdecking at the last minute means they won't know the deck well. Very often, the build you find online isn't optimal for the metagame you play in. Always playtest and modify it to your needs to prevent disappointments! If you modify or build the deck yourself, you will find yourself always in control. You will know what you need to win, you will know what you need to draw. In some cases, like me, you'll even know what you are going to draw next (or can guess to a high probability count). Building your own deck and knowing your own deck well will ensure you know what to do, when to do it, and how.
Rule no. 7: Every hand should be a good hand. Here's a well known rule. A lot of decks don't follow this rule, but most great decks do. They can practically run off any hand as long as it isn't a horrible hand. There are exceptions like Gifts where you have to mulligan for a usable hand against certain decks (Gifts needs loads of skill, unlike a lot of other decks). Stuff like ED, GW Glare, Heartbeat Combo, and so on can usually work on most of their hands. The most classic example would be weenie decks. Weenie decks usually drop a creature every turn, meaning that most of their hands usually contain a creature they can drop turn 1 or 2, which is always good. A deck built reliant on a god hand won't cut it (like Raging Goblin + Blazing Shoal).
Rule no. 8: Your deck should have a god hand. Some people call certain hands that you do draw a god hand. This hand is of course, a really good hand that will essentially guarantee a win if you play correctly. If your deck doesn't have a hand to call a god hand (in the cases of some decks, god hands occur far more frequently than not, which may skew perception of what a god hand is), if your deck doesn't have a god hand, it will most probably lose more often than it should. A god hand is generally the embodiment of your game plan using the deck. With Emminent Domain, a god hand will contain enough lands, a signet, and a few useful enchantments (most notably, Annex). Of course, a god hand is useless if you don't draw the right lands. You will want a perfect game plan. If your best draw is easily disrupted, you need to rethink your deck and game plan.
Rule no. 9: Listen to advice and debate on each point. Sometimes, you may come into advice on how to improve your deck. Do not take any argument 'as is'. Argue it out until you come to a good conclusion. If you can't, it means that both points are equally valid and relevant to the deck - and the point is a contention of personal opinion. Arguing it out usually let's you learn how to look at your deck from a different point of view and helps you learn deckbuilding.
Rule no. 10: Have fun building and playing the deck. Have fun while you're building and testing the deck. If you don't have fun playing the deck, build a deck that suits your playing style better. Some people can play any kind of deck, some people prefer control decks, some people prefer aggro decks, some people prefer combo decks, some people prefer complete rogues which are just plainly fun to play. While you do want to win with your deck, you won't play as well if you aren't having fun making good plays with your deck. If you aren't comfortable with some of decisions you have to make when you play the deck, don't play that variation of deck. Being comfortable with a large variety of decks makes you a much better deckbuilder and player in the long run. It just wouldn't make sense if a person who loves aggro is playing combo.
Some people might call me crazy for saying Skarrgan Skybreaker is worth looking at. It's very situational and very horrendously useless if you can't deal damage to your opponent before playing it.