It depends which area you are working in. If you are looking for a managerial role in an office then you would need some managerial qualifications, possibly a degree in business.
If you are wanting to apply for a secretarial role then you would need knowledge of word processing and be able to type at a fast speed. Some companies request that applicants should have experience of at least 1-2 years working in an office environment. If you are applying for the role of an office junior though, then this won't be a problem, and you can easily be trained up. You may need basic knowledge of other Microsoft programs such as Access, Excel and PowerPoint, and have higher GCSE grades in English Language and Mathematics.
You must have excellent communication skills, as you will be speaking with clients on the phone. In some cases, particularly with receptionist roles, you may need knowledge of how to operate a switchboard, though you may be trained up for this