If you would use the search bar to type in your question, or browse the FAQs, you would find several topics discussing this. But I will tell you here, to make it easy for you.
You can join a coven and become council. Some covens, like Spellcasters, automatically give you a council position. Once council you go to your covens home page, click the link at the top next to "Spellbook". Click the link next to "spells", and click the link that says "Add a spell".
You can mail someone who is a priest/ess or in a council position to ask them to add a spell for you and give you credit.
You can suggest a spell by writing it and posting it in the Spell Suggestions forum thread here.