How to appeal a refused Schengen visa?

Finding a lawyer is indeed the best course of action but we can't help you with that. In principle, your wife should have been informed of the grounds for refusal and the procedure to appeal.

Furthermore, since you are a EU citizen, if you intend to travel with her for a visit, there aren't many valid grounds for Spain to refuse the visa and a marriage certificate should be enough (see europa.eu). She most definitely does not need to provide the full Schengen visa documentation and fee but should really point out that she is your wife and you are an EU citizen. Even staying there to live with you long-term should be no problem, at least if you have a job or business in Spain. The Spanish authorities might simply need to be reminded of that. The EU might also be able to offer some assistance.

If she wants to stay there alone or you can't justify any income, it's a different matter.

First you should call to embassy and ask them about the reason of rejection.

In common visa can be rejected if there was some errors in documents or not all documents was served to embassy.

Also it can be rejected if your wife have some problems with the law.

The only way to understand what happened - is to call to embassy, or write them an e-mail. They will describe you what goes wrong and with this information you may better prepare to appeal.

By the way, are your marriage registrated only in Iran? If it's so, you also need some documents to approve your marriage. It's also possible to show correspondance between you and your wife and other evidences that you know each other for a long time and it is not a fictive marriage.

Good luck