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Quick clarification on D&D BECMI on how to hit, using THAC0.
TACH0 means "To hit armour class 0" which is your common reference for three reasons.
1: It sounds good.
2: You have a table of THAC0-THAC9.
THAC0: 19
THAC1: 18
THAC2: 17
THAC3: 16
THAC4: 15
THAC5: 14
THAC6: 13
THAC7: 12
THAC8: 11
THAC9: 10

Your base armour class is given by your armour type (No armour = AC 9, Leather = AC 7, Chainmail = AC 5, etc). then you add your bonuses (Shield -1 to AC, Dex 16 -2 AC). Let's say you have chainmail armour, 16 dexterity and a shield, you would have a base armour class of 5, which is modified to a 2 for your shield and dexterity bonus (5 - 2 - 1 = 2). Now you have an armour class worked, someone needs to hit you. Somebody has no bonus and decides to attack you. he rolls a 16 which hits AC 4 (THAC4 result). If he had 14 strength (14 str, + 1 to hit) then he would have modified his Roll to a 17, which hits AC 3 (THAC3 result). A miss in the first case, but in the second he would now cause damage. he hits you with a short sword, doing D6 damage for the weapon, plus 1 for his strength bonus (14 str, +1 to damage). He rolled a 4, giving him a total damage of 5. You now have a temporarily lost 5 hit points (keep your total HP and have a current HP).

3: you can calculate your to hit in another way. You roll a D20, add your bonuses and the enemy's AC together, if you would have hit AC 0 then you would have hit the target. You will not know the Armour Class of monsters in D&D, only the DM does. You are allowed to guess or figure it out, but each monster's stats may be different.
So how does this work in practise? You roll a D20, you get a 15. you then add your strength bonus of 2 for being strength 17 and get a 17. your opponent is in chainmail and shield for an AC of 4, so you add 4 to your roll. You get a total of 21, which is greater than your THAC0 of 19. You hit! Then you roll damage using a Longsword, rolling D8 for the sword and adding 2. you rolled an 8! you do 10 (8+2) points of damage and he drops dead.

Note: It is impossible to have an armour class of 10 or higher (even if they have a dexterity penalty for instance) but it is possible to have an armour class better than 0, and will be likely to occur in your second adventure and beyond.
Note: In hand to hand combat, you get a strength bonus to your to hit and to damage rolls. In ranged combat you get a dex bonus to hit only, damage is unmodified. Some thrown weapons use strength bonuses (such as hand axes).
Note: The modifiers for to hit are: Training, Strength, Magical bonuses, Miscellanious modifiers (Such as behind cover).

Combat menouvers at first level:
Retreat! Run 3 times your normal encounter speed away from the enemy. You give your oppoenent a free attack with +4 to hit and no shield bonus!
Fighting withdrawl: you back up 10 feet a round, able to fight the enemy as your withdraw.

Spears, pikes and certain weapons may "set" against a charge, dealing double damage.
Lances may be "couched" when charging on horseback, dealing double damage.

Please read the cyclopedia if you want to find more. Fighters gain fighting styles at 12 level (I think) for things like "parry". Also the weapon proficiencies give you certain skills or abilities as you level up. (on top of additional to hit bonuses and new damage for your weapons). You may advance in weapon mastery every once so often. Fighters have 4 skill slots at first and gain them more often, other classes get two to start.

Skills: You can get more as your progress.

I hope this helps.