It depends upon the degree of damage made by hacker and the amount of infected files. First step is to secure your admin credentials and scan all your files including your database using free file scanner like virustotal. Check also your hosting as shared hosting tend to get infected from other hosted site in the same server. If you happens to use a outdated or nulled plugin or wordpress theme, replace them with updated and premium version(buy from the author only). If you have an existing users, inform them of the situation and ask them to change password. Lastly try to install a good security plugin like wordfence, sucuri, wpcerber among others.
Identifying the source of hacking may help you avoid the second round.