“I would say something to the waiter.”
At a restaurant, your friend ordered a steak cooked medium-rare. But when he got it, it was over-cooked. He tells you about it. You say this to convince him to complain to the waiter.
I would say something to the waiter.
This is a shortened version of "I would ___ if I were you." It's a way of giving someone advice without directly saying "You should ___":
You should say something to the waiter.
This is a direct way of giving advice, but it can seem a little rude. "I would ___" is more polite.
When someone has a problem, a complaint, or a concern, you can tell them to "say something". This means to talk about their problem to a person who can fix it.
For example, in the subways of New York, there are signs everywhere telling people to report strange packages to the police. They have photos of bags or boxes sitting alone under a seat, and say:
If you see something, say something.
In this situation, "say something" means to tell a police officer or subway employee about your suspicions.