LP is the enemies HP, the initial value is always 8000.
The phases in your turn go like this:
Draw phase (draw a card)->standby phase(a phase to play specific traps and magic cards from facedown position on Field or activate certain monster effects on the field)->main phase 1 (play as many magic and trap cards as you have open spaces on in the magic trap zone, summon one monster in faceup or facedown, special summon as many monsters as you are able to)->battle phase(your turn to attack the enemy, first you attack the monsters, if monster is in attack position damage is compared atk to atk, if it's defense position it's atk to def. if a monster is in defense position you can't damage the enemy LP until it's removed from the field by attacking higher than it's defense. Lower attacks will damage your own hp 1200atk attacks 2000 def= 800dmg to the attackers LP. 2000attack to 2000def does nothing. 2100 attack to 2000def, destroys the defense monster. Fighting attack position monsters 2000atk against 2000atk kills both monsters no damage to LP. 1800atk to 2000atk kills the attacking monster. 2100attack to 2000 attack, kills enemy monster and does 100 damage to the enemies LP.)-> main phase 2 ( a recovery phase to play more magic cards or summon if you didn't in the first phase)->end phase(end turn).
I hope this helps you a bit