While kief tasty's advice is correct, this is still difficult for new players to grasp. Try running a 1/3 everything deck first, then start seeing what you need to fix. So in a 60 card deck, 20 creatures, 20 other spells, 20 lands. Again, this isn't set in stone but it's at least a starting point. Something to really look at is your mana curve and the amount of each card you play. Consistency is big, and running Playsets of cards can help your deck run more consistently.
You typically want more 1-2 mana spells, fewer 3-4 mana spells, and fewest 5+; the exception is running mana ramp decks to ramp to your larger cards. If you have more questions don't hesitate to ask, hopefully my advice helps.