I'm extremely terrible at interviews

First, don't say advices. Second, just be yourself. This stems from trying too hard and being too uptight so as to try and look the part. Wear the right suit, shoes, and groom yourself. Otherwise, just relax and stick to your preparation and what defines you.

I 100% think that if you do the following you will have no problem with carrying on a conversation (not just applicable to interviews).

Part one - read the book How to Gain Friends and Influence People (read it in a day, its not that long), this will give you a basis on how to get people to like you. One of the main themes is actually listening to what the person is saying and showing an interest in it, which plays a large role in having a conversation. There are also a ton of other things to learn about people in the book.

Part two - this is more about fixing your thinking on the spot problem. This is extremely hard to do and takes some balls, but I guarantee it will fix your problem. Go to a bar solo and just approach groups of people and try starting conversations and holding the conversation. You will fail miserably on a lot of groups, but you will eventually learn and get better. This is easier on campus bars since you already have something in common. Also, if you really cant get yourself to do it, take a few shots of tequila at first... and eventually try and do it sober. Note: you could also practice this in cocktails and any other place where people meet up basically... its just that bars is easy as there are always people there.

Good luck!

Prep like a madman. Not everyone is a natural but these skills CAN be learned.

Memorize a list of anecdotes and practice them in the mirror. You want to seem natural telling a story-not rehearsed

Interview for jobs you don't necessarily want. The lack of pressure will make you much more comfortable going in and you'll notice the difference once you're at ease

read interview guides to get the best answers to traditional behavioral questions and format them to fit your own experiences

Try not to freelance too much in the beginning (telling jokes, being overly friendly, veering off topic). That stuff comes once you've learned how to build a rapport, which should be instantaneous moving ahead. There are several great books on this, but the standard is Carnegie's "How to Gain Friends and Influence People". Also try Zig Ziglars "Secrets of Closing the Sale". Trust me, you'll want to know how to close.

I find that my best interviews have come when I took the approach of a Consultant working on an engagement. That means I showed genuine curiosity/interest about the role, asked plenty of operating questions to show that I have a sense of what to expect and what issues/problems they face, worked in some of my knowledge of the industry/company, and related it all to previous experiences. This will come with time, but if you take this approach rather than a combative one where you're trying to run out the clock or are just giving direct answers to questions and nothing more, it will show that you're engaged and adaptive.

Only a few people go into interviews "thinking on their feet". Most of us memorize scripts to the point that they seem natural when we spit them out. Do the same.