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One of the biggest questions I get when I encourage people to just speak, no matter how bad your grammar is and how few words you know, is “What will I talk about?? They'll just get bored and angry with me!”
This attitude is quite strange because it presumes a level of rudeness of foreign cultures that is (almost) never true. You'll find pretty much everyone is so eager to hear you speak their language, if you would just give it a try!
But ignoring the reactions, and just focusing on what you would talk about, once again I find it to be a weird question. What do you talk about in your mother tongue? Surely that will do! With a bit of imagination, you can work around lack of words and share something you are passionate about.
And if anything, that vocabulary should be prioritised in your studies over what many courses may consider “essentials”. Talking about travel and language learning has always been more essential vocab for me to focus on than learning colours has.
While there are many answers to this (it really depends on what you like to talk about), the simplest advice by far which solves many problems is this:
To be interesting you have to be interested!
Learn how to ask questions.
Let the other person do the talking.
This way you are not under as much pressure. You don't have to have a bubbly personality to keep them interested, you don't have to give long speeches, and you don't have to talk endlessly about yourself;
Ask them what they like – how is work going? What are they doing this weekend? Did they see the game yesterday? Do they like cats or dogs, and why? Who is their lifelong hero?
Think about questions that require answers much longer than a word or sentence. Something that opens up several minutes (at least) of interesting conversation.
When you ask good questions then they'll do all the talking for you. You can keep up the flow with clever use of conversational connectors, which helps the other person feel that they aren't waffling on uninterrupted.
I personally feel that the most valuable reason to learn any language is to learn about other cultures more directly, not to go around the world telling everyone how great I am. For this, you need to be a good listener, and that's way easier than being a good talker.
As well as being easier and putting you under less pressure, it gives the other person the spotlight.
Everyone likes a good listener! Several studies have shown that the most “interesting” people you tend to meet in social gatherings are actually simply those who do very little talking, but show genuine interest in finding more about who they are talking to, by asking the right questions.
So try it – don't know what to talk about? Ask the other person what they‘d like to talk about. The best way to be interesting is to be interested.