I send my step daughter $600 a month for school. can i claim?

No, that doesn't qualify as a tax deduction by itself. 

If your total support for your step-daughter is enough that you can claim her as a dependent, that is a tax benefit. Follow this link for guidance on claiming a dependent. 

If you can claim her and she has higher education expenses, that is another tax benefit. Follow this link for more on that. 

A step child is considered the same as a biological child for tax purposes.  That is, if the child lives with you when she is not a school, you may claim her under either the "Qualifying child" rules or the "Qualifying relative" rules. 

See full dependent rules at: https://turbotax.intuit.com/tax-tools/tax-tips/Family/Rules-for-Claiming-a-Dependent-on-Your-Tax-Return/INF12139.html

You may only claim the education credit if she qualifies as your dependent.  If she is not your dependent, the parent that does claim her as a dependent can count the money you sent in claiming the credit for themselves.