Good questions, and I am happy to answer.
1. Creativity, ingenuity, intuition, and faith. A spell is simply a physical manifestation of an intent - it is letting an idea that you wish to push out into the universe be known through a physical and spiritual means. To create your own spell, I suggest you look at other spells, and recognize their structure, their needs, and their energies. Start to experiment, and get creative! After a while, you will know yourself well enough to trust in yourself, your intuition, and you will generally know what you are doing. :)
2. A charm is an object energetically charged with an intent - typically something worn or kept on the person as the object's purpose was to usually serve the person after being 'charmed'. They work just as all sigils, or amulets, or enchanted objects work. They are charged with energy specific to the caster's intent and it work accordingly (if you made a charm - a ring you like to wear - for vanity, wearing the ring may make you do your hair in the morning when you normally wouldn't or put on some more make-up, more than usual). I am assuming that when you ask "How do we put a spell on it?" you are wondering how to create a 'charm'. Well, I wrote about this a few hours earlier actually. here is the link:
3. The spells here could be posted by... just about anyone. This is the internet and it is a public website. I advise caution when reading the spells here. Some are known to be false. A good section to avoid is the 'Fantasy Spells' section, as they are all fantasy and will really do nothing for you or your practice. On top of that, I suggest you write your own spells (which brings us back to answer one of question one).
I have cast many spells in my lifetime, and nearly all of them with results. Magick works, it simply works. If it didn't I don't think we would be here.
Hope you enjoyed my answers.