You have to keep two things in mind. 1 - Normally, you only get one Combat Phase, 2 - All monsters attack at the same time. Untapping a monster after it has attacked will in no way allow it to attack once more, because you are past your Combat Phase. Even if you were still in the Combat Phase, all attacks are declared simultaneously, and you are past the 'declare attackers' step.
The ability "Defender" is actually a disability, a creature with defender cannot attack. Creatures with Defender often have "Wall" as their type.
No, unless they have haste.
Vigilance just means the creature does not tap when it attacks. It can be tapped by other spell or abilities, or as the cost for an ability that requires tapping - and must of course be untapped for it to be able to even declare an attack.
Creatures with Flying may block creatures that don't. Note that in Magic, creatures do not 'attack directly' as such, they are declared as attackers, and then creatures may be declared to block them. The creature with Flying in this case, can block creatures with, and without Flying.
You can't attack an opponent's creature directly. you attack your opponent directly with your creatures. It's up to them to decide whether or not to block. The exception to this is if one of your creatures has the Provoke ability which means you can force another creature to block it.
yes,they can!
Creatures with Wither deal damage as -1/-1 counters instead of usual damage. So in the case of a creature with Power 3 and Wither, instead of dealing 3 damage to another creature, it will place three -1/-1 counters on it instead.
Creatures are not brought into play tapped, but they have summoning sickness preventing them from tapping to attack or to tap to activate any abilities unless they have the keyword "haste".
Creatures cannot be 'attacked' in Magic the Gathering. Players are attacked, and creatures are declared as blockers, although there are a few special abilities that can force things to attack or block. A tapped Planeswalker can be attacked, as per the usual rules for declaring attacks against Planewalker cards. Targeting a creature with a damage dealing spell or ability is not 'attacking' them. The creature being tapped has nothing to do with its targeting eligibility, it can still be targeted by cards that deal damage. Damage spells never target Planewalkers directly, the cards target the controlling player and the damage redirected to the Planeswalker from there. This can be done to a tapped Planeswalker.
An 'attack' is what you do during the combat phase, you declare attackers by tapping them (or just declaring, if the creature has Vigilance) and these are now 'attacking monsters'.Tapping to use activated effects, even damage dealing ones, are not attackers, and the damage is not 'combat damage'.
Defending creatures deal damage to an attacking creature's toughness when they block equal to their power. It's called 'blocking', not 'attacking back'. Summoning sickness has no affect on blocking. Summoning sickness is the inability of a creature to use tap (tap symbol) abilities or attack until the next turn of the owner who summoned it.
VigilanceVigilance is a keyword of an ability that already existed before it was keyworded, as far back as Alpha with Serra Angel. Creatures with Vigilance do not tap to attack and are mostly white.