As an international graduate applicant, I'd contacted professor that I am interested to work in his research. He replied that I should apply to school first and if I get admission, then contact him? Is he aware that I want to join as an graduate assistant? What does he really mean?
In the US and some other places, graduate (and other) admissions is handled by the university itself in coordination with an academic department. They are not under control of individual professors. Therefore, in such places, you need to deal first with the admissions system, and than with a professor.
Depending on how you phrased your letter he may just be informing you of the rules.
You could, of course, send a letter to professors expressing your interest, telling them that you are applying through normal channels and asking if they are considering taking on students for advising. Such a letter should show some knowledge of the research interests of the professor and your own matching background.
For your specific question, I think the professor is being as positive as he can be. Don't take it as a negative.
Also, depending on the field, the professor may not be responsible for choosing graduate assistants. Again, it may be a university or department decision. In some other fields, the professor, if grant funded, can staff his/her own labs. In mathematics, for example, while you have a specific advisor, you may 'work for' the department, not that professor.
From what I can read, he is neutral about you.
In most institutes this is a standard procedure: individual faculty member cannot guarantee admission or post-admission employment. This is to avoid providing mixed messages to the applicants. If the admission committee denied the application, but the applicant came back and said Prof. so and so said he would hire me, many complications can happen.
Second, most assistantship within a school is prioritized for students. So, in order to be even considered, the candidates usually would have to be admitted first. Without that it's pointless to discuss even you have an excellent resume.
Third, be aware that international students may face extra barriers when looking for employment. Within school jobs such as teaching or research assistantships are generally of "equal opportunity." Jobs outside the department that go through a formal hiring process can also impose extra measures to ensure local citizens have been considered first. All in all, make sure you have the means to at least get through the first year, don't count on the plan that you will get a job as you arrived.
Perhaps instead of e-mailing individual professors, consider e-mailing the admission personnel as well as the career service personnel of the department. They are more informed about how internal hiring is done. They also have the incentive to attract more students, so they are more likely to engage and follow up with your inquiries. Ask for a spreadsheet of estimated costs of living and study: they are immensely helpful for financial planning.