You probably don't want to be told that you ought to get a lawyer. But you should. Learning how to prepare a witness for deposition, and then how to represent the witness during his deposition, is one of the most difficult parts of practicing law. I've been doing it for 30 years and I'm still, true to the phrase, "practicing" it.
Assuming that the defendant is represented by counsel, you should expect him or her to ask you very probing questions about every part of your case -- including, at a minimum, your claims, the defendant's possible defenses, your claimed damages, and so forth. You'll be asked some general background questions as well. Defense counsel will likely be courteous and respectful, but make no mistake: His or her duty is to zealously represent the defendant, not you, and if defense counsel can get you to make admissions that damage your case or even blow your case out of the water, that's exactly what he or she will be trying to do.
There's no necessary or inevitable connection between depositions and settlement. Sometimes there are settlement discussions before or after, but often there aren't. The fact that the defendant has committed to investing lawyer time and also the considerable out-of-pocket expense for the court reporter (and possibly videographer if it's been noticed for video) means they're spending money on something other than paying you. But if you do poorly at your deposition -- and frankly, most pro se plaintiffs do terribly -- that is going to drive the defendant's perception of the case's reasonable settlement value down.
For those reasons, you might want to consider exploring settlement prospects well before the deposition. Of course, that may also signal that you're nervous, which might cause them to tighten up and plow ahead with the deposition.
The best thing you could do would be to get a lawyer.
Get a lawyer now. You don't know the rules of evidence. You don't know the rules of procedure. You are going to be at a severe disadvantage because you will have to both answer the questions and make objections. I would never attempt that and I am a litigation attorney. Having said that and assuming you are going to ignore that advice you should: Always tell the truth. Listen very carefully to the question asked; if it can be answered yes or no then say that and shut up. Do not volunteer information. Do not explain answers if you are not asked to. Do not educate the attorney asking the questions. You are not there to convince anyone of anything. You are there because they noticed you. Get it over with as quickly as possible. Now, re-read the beginning of this.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.