No, they do not. It would not affect them in any way though, the only Planeswalker affected by tapping is Gideon Jura.
No. Permanents only enter the battlefield tapped if they specifically have an effect that says they do, such as Nevinyrral's Disk. No card type inherently enters the battlefield tapped.
No, note it says 'Untap target nonlegendary creature", not "Untap target tapped nonlegendary creature." Most of the spells that steal control like this, will allow you to untap the creature if it's tapped. But they're all still fine to be used against creatures that are originally untapped too.
Go and tapped people head and s..t on them
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".
Of course. There is nothing about being tapped that stops a creature being targeted or otherwise affected by a spell or ability, unless specifically stated that they work only on untapped creatures.
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.
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.
Ninjutsu [cost] (cost) reveal this card from your hand return an unblocked creature you control to its owners hand put this card into play from your hand tapped and attacking
No, of course not. Summoning Sickness stops a creature from attacking or using an ability with a Tap symbol as a cost. It does not stop it from being tapped by other effects, or for certain costs which require a tap, but do not use the Tap symbol (they can be used for Convoke for example) and they are certainly not destroyed if they end up tapped.
no
You choose a player to use it on when you cast it. When it resolves, tap all their creatures. When that player next has an untap step, he may not untap any of these creatures. A creature played and tapped after Sleep resolved, may untap in the next untap step. However a creature that was already tapped before Sleep resolved, may not untap. That's because Sleep's effect was applied to it (it affects all their creatures, not just untapped ones), even though it could not physically become tapped by Sleep.
No, you do not. That is what's called an 'upkeep'. Under normal circumstances once you've paid the equip cost, the equip will remain on the target permanent until either is destroyed or the equip is moved to another card.
Summoning Sickness stops a creature from attacking, and prevents it from tapping as the cost of an activated ability that uses the tap symbol. Non-tap abilities are not affected by summoning sickness. They can be tapped for the cost of abilities that do not use the tap symbol (such as that of Gilt-Leaf Archdruid).