Starting a conversation and keeping it going is sometimes a problem on first or second dates if you hardly know each other or you're nervous. When we get excited, our brain tends to shut down and we can't think of anything cool to say. It's perfectly natural. The best thing you can do other than just to relax and stay in the moment, is to prepare some things you can talk about before your date.
Where can you get some ideas for things to say? If you met someone online, take a look at their profile and see what they're interested in. It's easy to get someone talking about something they like. In fact you might be sorry you mentioned it. Sometimes people ramble for a long time. But at least it takes the pressure off of you to find something clever to say.
Make a written list of everything you know about your date. Think about open ended questions that will get them talking for more than a few sentences. Avoid asking questions that can be answered with a couple of words. Practice asking questions that require someone to give you a very long answer. With practice you will get very good at this skill, and you won't need to talk much on dates. Of course if the other person is doing most of the talking you can learn more about them and you don't need to worry about what you should say.
When some people get nervous they talk too much or say things they shouldn't say
Talking about inappropriate topics with someone you don't know well enough
Talking about things that make you or your date nervous
Being too quiet
Body language - not making eye contact, crossing your arms, nervous habits
Distractions to what you're trying to say like bad breath or cheap cologne
Why did I mention body language? Because most people talk a lot more with their body than they do with their voice. Think of your body as picture people see. A picture says a thousand words. You might be saying one thing but your body might be saying something totally different.
For example if you cross your arms during a date, your body language is speaking loudly. It's saying that you're not interested and want to be left alone. That's enough to make anyone uncomfortable around you and at a loss for words. On the other hand if you lean into your date slightly as they are speaking, it tells them that you're paying attention and interested. They are more likely to tell you more and there will be fewer moments of dead silence.
So make sure your body language is conveying exactly the same 'words'. Conflicting messages will confuse your date and it might turn them off. The best way to naturally say the right message is to be 100% focus on your date. When you take an interest in someone and genuinely like them, your body language will usually be in alignment with what you're saying verbally.