Well, if you want to build a deck here are some steps.

1: Choose a theme! Good decks always have a theme, Zombie decks rely on huge power from the Castles, Dragons are powerful themselves, and toons are 95% impossible to be destroyed.

Here are some good themes:(around 10-18 cards for theme)

Note: don't use gate guardian, like everyone metioned, it takes 9 tributes. Include his pieces instead, their effects could destroy gate guardian him self.

Zombies

Dragons

Beatdown

Harpyies

Fiend/Spellcaster

Monster power up

Exodia

And when the japenese sets come out:

Gods

Toons

2. Find weaknesses your deck has, like low defenses or attack or monster/Trap/Magic removal. Fix them with cards that counter them.( 3-10 Cards)and make defense against commonly used cards like:Dark hole,Monster Reborn,Change of Heart,Man-Eater bug,Card destruction,sword of deep seated, mirror force,seven tools,magic jammer,heavy storm,trap hole, and many more.

3. Add some ovious good cards:(5-15 cards) 

La jinn

Magic Jammer

Seven tools of the bandit

And the restricted list

4. Don't make your deck too big! 40-50 Cards. If you have too much, you won't draw what you need. Eliminate all cards you don't need. Don't include cards that you put in for nothing, like Saggi the Dark Clown, he'll only be useful once the Crush card comes out.

5. Test it Out!! don't go to a tournament without testing it. If you have no one to duel, make another deck and duel your self.