You need the server software first. Once you get the server set up, you can copy your save to the server.
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Minecraft has a separate multiplayer server file to download for free. Just look for the links for the forums or look at Youtube to find out how to set it up.
Custom maps? You will have to setup a server, download the map and replace "world" folder with the custom map (Must be renamed to "world".) Or you can play on my server and we can set something up. I will setup world file and all that. Just tell me the map and your IGN! (In game name.) Email me at: dave_robertson@mail.com
You can set TNT off in Minecraft Beta by clicking it, or by triggering it with redstone.
No, Minecraft Realms are 100% vanilla as they are provided by Mojang.
Yes. Macs and PCs can play together also. Whoever is going to host the server has to download the multiplayer server software to set it up for the other person. There is plenty of information on how to do it. It's not too hard to do but if you just look online for something like the "MinecraftWIKI" site it has step by step instructions. It's even easier if your both in the same house as Minecraft doesn't need extra software to play a multiplayer LAN game.