What can I do if my manager constantly insults me and makes fun of me?

There may be nothing that you can do about it. Without more specifc details of your situation it's hard to say. However, for an employee to bring a claim for  a "hostile work enviornment", there must be more than harassment taking place. The actions of a co-worker must prohibit them from adequately performing the duties of their job. Additionally, such harrassment must be based on legally actionable discrimination (or retaliation). This means that their treatment cannot be based on therr being a member of a "protected class". In other words, they cannot be singled out for different treatement at work based on their race, religion, gender, sexual orientation, nationality, age or disability.
Basically, most employment relationships are "at will". This means that an employer can set the terms and conditions of the workplace much as it sees fit. So unless you have protection based on the above or the terms of company policy, a union agreement or employment contract, you don't appear to have a case. While seemingly unfair, your choices may just be quitting, putting up with this treatment or standing up for yourself and risk termination.