I am 48 year female (us citizen)3 kids all well settled in us.right now I am visiting friends and family in united kingdom, I want to stay here for 3months but to support myself financially I need to get a part time job. How do I do that

You need a work Visa... or you will be violating the law. And they don't just hand those out...

You will be there as a tourist with no legal right to employment without that Visa.