He is saying you need to check whether you would be accepted into the school first. This information is probably on the school's web page. Possible things they might require are a certain level of grades on a relevant undergrad degree, letters of recommendation, maybe you need to be a citizen or landed immigrant in the country or you will need to pay foreign student fees, etc.
The school should also be able to give you advice on the normal process of entering a PhD program at the school. Admissions, deadlines, fees, applications for various things, etc.
Once you have navigated those hoops... He is saying he would like to talk to you about the idea you have for research. He has not committed yet. It may be that you can convince him, depending on how the conversation goes.
So, first, check that web page. If you don't find the information, dig deeper. Maybe you will need to find who is the admissions representative at the school. Maybe you need to email them. Maybe there is a phone number on their web page. Get that information. If it looks like you will be qualified to enter the program, then go on.
It may mean you need to provide transcripts, letters of recommendation, etc. If you have some kind of scholarship, grant, fellowship, etc., that is a huge bonus to admissions.
NOTE! Many scholarships do not consider you unless you apply. So find out from the school what scholarships exist that you could apply for. A few $thousand extra in the bank is usually not a bad thing for a PhD student.
Schools usually try to make some kind of financial arrangement so that you can at least pay your rent and other living expenses while at the school. But you may wind up with some debt after your degree. You should figure that out in advance. If the debt you will come out with is too much, possibly you need other plans.
Then contact the prof and try to set up an interview. Maybe the first one is by phone or skype or something. If that goes well, maybe you have another interview in person, possibly with more people there than just the prof. Maybe you meet other people he works with, his other students or his post-doctoral fellows or research associates, etc. Maybe you meet some other profs who work with him at the school.
The in-person interview is likely to happen only if you are accepted, or extremely likely to be accepted, at the school.
Maybe you and he don't match and the result is no. Maybe you and one of the other profs are a good match, and the other prof suggests you join him. Or maybe this prof suggests you join his group. If you can, you should make up your mind in advance if joining any of these folks is good for you. And you should be keeping your eyes open during the interview for indications that you would or would not be a good fit there.
Try to have a positive outlook. To prep, try to look up some of the recent papers of "your" prof, and maybe his work group. And maybe some papers from other profs in similar research areas at the same school. You don't need to be expert level on them, just be basically aware of what research they are doing recently. Especially examine papers that might be related to the research you want to do.
If it's a basically physics thing, then arxiv.org is an excellent resource to find research by a particular prof. There will be similar resources for other topics, but I am unable to point you at them. Google is your friend.
Hope this helps.