Where to file lawsuit when borrower is in DE and lender is in NJ?

You can file in either state, but usually you would file in the jurisdiction most convenient for you (usually your home state) if you can get jurisdiction over the defendant there.  As your borrower entered into a contract with a NJ resident which was to be performed in NJ, he has sufficient contacts with NJ so that it would have jurisdiction over him, even if that is his only contact with NJ. Not sure what you mean about the "filing process"; filing the summons and complaint takes a minute.  Service of process and litigation of the case would take much longer, but I can't give you a good estimate because it depends on whether and how hard the defendant fights, and whether he has any meritorious defenses.  Theoretically, you could serve him easily, he doesn't contest the case, and you could have a judgment within 2 months.  Alternatively, he could duck service, answer the complaint, assert defenses, go through all sorts of discovery and motion practice, the case could go all the way to trial, and you might not get a judgment for years.  It is unlikely that he would do so because (a) it is very expensive;and (b) from the little you've written, I don't know what his defense might be.  However, it is possible. You can't file a criminal lawsuit; criminal actions are not brought by private parties, but rather by governments.  You can go to the police or local District Attorney and seek to have them press charges, but it is unlikely that they would, because breaching a contract is not a crime.  Theoretically, if the person lied to get you to loan the money and never had any intention of paying you back, it could be some sort of crime based on fraud, but that is very difficult to prove beyond a reasonable doubt.  Cases like this are generally considered to be private civil matters, not criminal.