The obligatory parts
Under article 189 of the Civil Procedure Code: the name of the court; the full details and the addresses of the claimant and the defendant; the exact demand of the claimant; the value of the claim (if the claim needs a valuation); the facts of the case and the evidence for them; information about the pre-trial procedure, if the law or the contract sets one; a list of the attachments; the signature of the claimant or the representative. It also helps to give the legal basis and the date.
Write the demand exactly
Not "restore my right", but "recover from the defendant 15 000 000 so'm of principal debt and 1 200 000 so'm of interest for late payment". As a rule, the court decides the case within the demands that the claimant states.
The attachments
The contract, the payment documents, the correspondence, the acts; the claim and the proof that you sent it, if the law or the contract sets a pre-trial procedure; the document about the payment of the state duty and the postal costs; copies of the statement for each defendant.
Frequent mistakes
Filing with the wrong court, non-payment of the duty, no calculation and an inexact address of the defendant. Each one delays the case. The judge returns a claim that this court cannot hear, that does not meet article 189, or that has no document about the payment of the duty. After you correct the defect, you can apply again.