The best time to start a conversation is when you are next to someone who is not moving. You can't start talking to someone walking away from you. Then you are bothering them by making them stop. But if you are sitting down on a bench and someone is next to you, or you are at a beach and someone is close by, you can start a conversation by saying something generic. Here are some conversation starting sentences. But after you make your first sentence, you should see by their reaction if they want to continue the conversation or not. If they appear a little rude, then stop talking to them. You don't have to say anything to stop talking. They don't expect you to. They expect you to not say another word. But if they ask a question back, and they seem friendly, then you can continue with the conversation.

"The weather is so nice today. Are you from around here?"
"How is your day going so far?"
"There are a lot of people out here today. Is this normal?"

An easy way to start talking is when you notice something such as their kids, family, or a dog they might be with.

"Are you here with your family?"
"Are you on vacation?"

"Are those your kids? They're so cute?"
"What kind of dog is that?"

It's much easier if they start the conversation. If they make a comment or ask a question, then you know that they would like to talk.

Finally, it's common for these types of conversation to last only a couple of minutes. So don't feel bad or feel pressured to keep the conversation alive.

There isn't that much more to tell you about speaking with strangers. The best way to understand what types of things are said in a stranger to stranger conversation is to hear a couple of examples. I will give a couple examples to help you understand more.