The first paragraph should state that you are resigning from the company and the effective date of your resignation. Do not forget to include your position in this paragraph.
You can state in a sentence or two of how thankful you are to be given the chance to work in their company and acquire new skills. This paragraph is optional but it helps to leave a good impression to the company as you made need them for future referrals. Instead of thanking the company, you can also write the reason for your withdrawal.
You can conclude your resignation letter by telling the company that you are, in any way, ready to help them in the transition that will happen to ease the inconvenience that your resignation may have caused to the company.