Dear Sir/Ma,
I was excited to see this job post and it is my superb pleasure to apply for this job because I have experience in this job.
I have many years of experience working in different organizations. Recently I  worked as office assistant with Dean of Social Science, University of Abuja. The job exposed me to all the responsibilities I will be taking as Office Assistant in your organization. 
I have different skills that has qualified me as the best candidate for this job. My CV can attest to that.
With the above information I'm confident I'll given this job.
Thank you.