Starting with the second question first:
Whenever you see an ability like " (something) : (something else)" with a colon, it's what is termed an "activated ability". The thing on the left is what you have to do in order to use the ability. In this case, it means that as part of the cost of paying the ability you have to tap the land. The thing on the right is the result of the ability. In other words, "(tap): Add one green mana to your mana pool" means: "If the land is untapped, you can tap it. If you do, add one green mana to your pool".
But you can ONLY use the ability when the land is untapped. If it is tapped, like Khalni garden when it comes into play, then you can't tap it for mana. Conversely, if you have some way of untapping a land during your turn (think Arbor Elf, for example), then you can tap the same land for mana twice in one turn. The rule to remember is that each time you want mana, you have to tap the land.
In general, "comes into the battlefield tapped" ONLY restricts you from using abilities that require tapping. The same is true for the "summoning sickness" mentioned in a prior answer, which only effects "tap" abilities and attacking. For example, if you have enough mana out, you can both play Merfolk Seastalkers (a creature with the ability 2+Blue: Tap target creature without flying), and immediately spend the mana to tap a creature. You could even use the ability if for some reason the Seastalkers entered the battlefield tapped.
One other correction to an earlier answer: You no longer lose life for having unused mana in your pool. However, the mana disappears from your pool after each phase of your turn -- you can't tap a land for mana, attack, then use the mana later.