Well, it depends on your language level in the practice languages. When I first started trying to chat in French, I couldn't do much more than respond to very basic questions and even that was a stretch. Hopefully you have very understanding language partners and you both really want to help each other.
One thing that you can do is plan ahead before the chat and suggest topics that you would like to try and discuss. Hopefully you have also made it very clear what your level is. If you go in knowing roughly what to talk about, you can at least look up a little vocabulary ahead of time.
If you run out of topics quickly, you can try some open ended questions. i.e. do you have any stories from your childhood that you would like to share? Can you describe the town where you grew up? What was your favorite meal last week?
In other words, try to get the other person to talk about something that let's them go into as much detail as they can. Don't constantly interrupt your partner, but do try to ask some questions based on what they have told you. It's good practice for both of you to confirm that you are understanding what they are saying. It's also a great way to branch off into other topics.
Example:
A) What was your favorite meal last week?
B) I had pizza on Tuesday, it was great.
A) Oooh , pizza. I love pizza too. What was on the pizza?
B) Bacon, onions, and pepperoni. What do you like on pizza?
A) All kinds of things. Once I made a pizza with chicken and french fries on it!
B) No way! What made you think of that?
.... and on it goes.
Even with a very basic level of vocabulary, you can talk for quite a while about simple stuff. Also don't be afraid to throw in words in your native language if you don't know the target language word. Hopefully your partner will help you out with the vocabulary you don't know and vice versa. Have google translate handy just in case :)