How can I make this sentence better for my objective on my resume.

To obtain an administrative assistant position where my professional skills will be used to the fullest.

Do you HAVE to have an objective statement? Usually those are pretty lame and unnecessary.

http://owl.english.purdue.edu/owl/section/6/23/
Check out Purdue University's section on professional writing, including resumes and cover letters.

If you absolutely have to include this unnecessary item because of a class assignment, start out like this:

To work as an administrative assistant ...

What are those "professional skills"? Exactly what do you mean?

What does "the fullest" mean? It's a very awkward phrasing and is just about meaningless.

You'd be far better off to include a section called Core strengths or simply Strengths in place of Objective -- and then bullet-list those strengths in detail. Here's an example:

~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Occupational Expertise

Interaction with customers, in person and via telephone
Communication with staff
Computer use:
~ Basic Word and Excel
~ Data entry
~ Internet: email, searching, web-publishing

Operations management
Problem solving and conflict resolution
Ordering and inventory
Budgets and cost controls
Profit and loss statements
Staffing
Payroll