If I'm renting a room from friends but I'm not on their lease, does that make them my landlord? should I have them sign my rent certificate? Can I file as head household?

You cannot file Head of House hold unless you have a closely related person (such as a child) that you support as your dependent and you pay more than 1/2 the entire costs of maintaining the household.

As far as your other question, unless you have a formal rental agreement with the friends, you are probably just sharing expenses.    If you need something formal, they should first find out if they are allowed to sub-lease in their rental agreement.

Head of household can only be filed if you can claim a dependent (family member or in laws ) that cannot provide for themselves. You have to have paid at least 50% or more of all the bills for at least 7months to a year with that dependent living with you. Bills include groceries, rent/mortgage, utilities, etc..