How do you make money fast in fallout new Vegas?

There are a number of ways to make money in Fallout: New Vegas.

I found that the fastest way to make money when you are beginning the game is to play Caravan. When you learn the rules (it can be a bit tricky at first) it is quite easy to win practically every time. My own strategy is to add all the 10's, 7's and 9's (to make the desired total of 26) to your deck, and then add most of the face cards to sabotage your opponent's caravan or change the direction of your own.

Another way to make money fast is to play Blackjack on the Strip (if you can make it past the Deathclaws with low level, you can travel here at the start of the game). The rules are simple, so it is a matter of Luck (a S.P.E.C.I.A.L attribute) and quick saving!

If you find all of the snow globes (easy enough to find with a guide), you can sell these to Mr. House for a total of 14,000 caps.

Another obvious way to make money is to explore areas not directly involved in the main storyline. If you focus on only high-value items, it can be considered a reasonably quick way to make money. Look for enemies who carry expensive armor and weaponry (e.g. Incinerators, Grenades, Missiles, Assault Carbines etc.) - such as the Super Mutants on Black Mountain - as merchants will pay a lot of caps for these items. Increasing your barter skill will also increase the amount of money you get for each item.

You can do all of the above and retain good karma. However, if you a playing as a character with bad karma, making money fast is much easier as you can steal from and kill "good" people for high value items. If you are playing this way, stealing from the Silver Rush, killing the Gun Runners, and the Brotherhood of Steel is a good way to make a lot of caps quickly. Kill the BoS in their bunker, take the Long Haul Perk (which allows you to fast travel when you are overencumbered) as all the weapons and armor will be very heavy, repair them to a good level, and sell to a merchant with a lot of caps (e.g. Vendatron). This is a particularly good method, as they will respawn after a certain amount of time and you can repeat the process.