Do you mean you want to be a trustee (ie on the governing body) as well as being an employee of the charity?
It's quite a complicated subject, but in general trustees of charities are not paid for being trustees. In some circumstances, trustees may be paid either (1) for just being a trustee or (2) for being a trustee (for free) and working for the charity (for example, as marketing director). But these are special circumstances - can give you more information on the general rules governing this if you want.
The whole thing is regulated by the charity regulator, the Charity Commission, whose advice on payments to trustees is here.
Your options are:
Get some other people to be trustees of your charity, and be a paid employee.
Presuming your charity is a brand new one, set up your charity's governing document so that it allows trustees to be paid employees as well.
Set up the charity so that you are a (unpaid) trustee, but not an employee.
Get the Charity Commission's permission to have you as a paid employee as well as a trustee. They would need a very good reason.
Hope this all helps and do post again if you need anything else! (I used to work for the Charity Commission and have also worked for government on the changes to charity law in 2006).