I have Grand Theft Auto 5 and I seem to be really low on money. I've looked up how to get money but I couldn't find anything but online money. So does online money transfer over to my single player?
The short answer: no.
The somewhat longer answer: no, but there are a myriad of ways to acquire money in single player GTA V. One of the more notorious ways is utilizing the assassination missions and the stock market to make massive amounts of profit from the shares' fluctuations. One such guide would be USgamer's article "How to Make Big Money in the GTA V Stock Market". A simple google search for "gta v make money" reveals a myriad of guides for this specific method.
IGN also has a more thorough guide detailing multiple methods of obtaining currency in their guide "How to Make Money in GTA V"
The bike thief random event in Rancho gets you $100,000 in stock later in the game when you return his bike, which you should sell immediately once he texts you. Just show up on Little Bighorn St. around 5 a.m. after doing Tonya's first freaks mission. Then the sunken barge is not far from there- no scuba gear needed for that one. You can also do this after Franklin & Lamar. Go down to the sunken barge in Rancho. Get the package(briefcase), then swim to the stairs. Exit the water, then quicksave and restart the game. Slowly swim back and the case should be there within 30 seconds from the time you reload and start. Keep doing it till you are bored. I made 2 pickups and could have done more, but staying in a certain time frame.