Another player once told me of the rule of four. That means take nine cards and get four of them, then fill the rest of the 60 card deck with land, and that's your deck.
Figure out which creatures do what best for you, and how much mana they cost. Then use them and build your deck around them.
Don't rely on cards that you have only one of unless it's a legendary that really makes your deck shine and assists what the rest of your deck is doing.
Focus on what you want your deck to do. Do you wish to protect yourself with massive lifegain amounts and then pull of a sneaky win with
Felidar Sovereign? Do you wish to kill your opponent's entire army, constantly, while protecting your own? Do you want to run right over them quickly with an unstoppable army of flying creatures, or would you rather slow their army down, freeze them in place, and slowly build up your game?
If you're going to play creatures that cause things to happen when they enter the battlefield, such as tapping an opponent's creatures, granting you land by letting you search for it, or any other ability (two excellent ideas are
Pilgrim's Eye and
Sunblast Angel), then get
flicker creatures so you can keep bringing those cards back and using the ability over and over again.
Whitemane Lion and
Emancipation Angel are excellent examples.
If you're running tokens,
Growing Ranks really helps your tokens grow.
Anointer Priest is good for token decks, though don't overlook
Soul Warden,
Soul's Attendant,
Suture Priest, and
Healer of the Pride.
If you're more interested in blue/white,
Ephara, God of the Polis, is hard to beat. She's a weak god but has powerful card advantage. (That means you can draw constantly.) Cards like
Stealer of Secrets and
Eternal of Harsh Truths supplement that strategy. If playing blue, never overlook
Elgaud Shieldmate. Your best creature gains partial protection from enemy spells.
Or maybe you're interested in playing blue/white spell return so you can keep a constant damage defense going? Sun Titans and
Scribe of the Mindful make a good mix, and
Riot Control would be most helpful. You simply use a
Riot Control spell when your foe attacks you, then sacrifice a
Scribe of the Mindful to get it right back into your hand. After that, counter-attack on your next turn with
Sun Titan and get
Scribe of the Mindful back.
And I'll give you some advice about
Tromokratis.
Never attack or block with her.
Always use the artifact
Dragon Throne of Tarkir on her. She keeps her hexproof, and gives your entire extra power equal to her impressive numbers until end of turn, either for attack or for defense.
Hope that helps.