A turn in magic is split up into 5 phases. Beginning phase, pre-combat main phase, combat, post-combat main phase, and end step. In the beginning phase you first untap all the cards you control on the battlefield(perminants) then you have your upkeep. Most of the time nothing happens during your upkeep but some cards have an effect that triggers on the upkeep. The last part of the beginning phase is the draw step. During this part you draw a card from the top of your deck on every turn except on the first turn if you went first.
On your main phase you can play lands, creatures, sorceries, enchantments, artifacts, instants, or planeswalkers. You do After you do what you want in your main phase you move to combat.
Combat is split into 3 more parts. Declare attackers, declare blockers, and damage. On your turn if you have creatures that can attack (They are untapped and they don't have summoning sickness) when you move to combat you tell your opponent which creatures will be attacking. You also when you attack you also tap your creatures. Then once you have declared all attackers your opponent will declare which creatures he or she wants to block with. They can only use a creature to block if it is untapped. Most people show this by putting the blocking creature up against the attacking creature. Then once your opponent is done you go to damage. There are two part to this first strike and regular strike. Most creatures deal damage during regular strike and all at the same time, so whatever creatures you are attacking with will deal damage to either the creature(s) blocking it, the planeswalker it is targeting or the player it is targeting. If a creature is dealing damage to another creature you subtract one creatures power (the first number) from the others toughness (the second number) and visvers for the blocking creature so both will have been dealt damage. For a player you just subtract the power of the creature from the players life total. For planeswalkers you subtract the creature's power from the amount of loyalty points it has. Some creatures might have the keyword ability first strike or double strike. First strike means that it will do damage first before regular damage and double strike means that it will deal first strike damage and regular damage.