Start with your thesis statement- what point are you going to make about same sex marriage? The best thesis statements are ones that can be argued with. So "Same sex marriage has become a big issue these days" is a weak thesis statement. It can't be argued with, so it doesn't need to be supported. Do a little reading on the opinions on both sides, and then decide what stand you're going to take.
Start with your thesis statement- what point are you going to make about same sex marriage? The best thesis statements are ones that can be argued with. So "Same sex marriage has become a big issue these days" is a weak thesis statement. It can't be argued with, so it doesn't need to be supported. Do a little reading on the opinions on both sides, and then decide what stand you're going to take.