I think this has been reiterated many times around here and elsewhere, but I cannot stress how goddamned important this is.
Do NOT fly your ship if you don't have enough rebuy for it.
New players wondering wtf rebuy is: rebuy is ship insurance, it's how much you pay to galactic insurance co. if your ship gets destroyed for whatever reason. It is shown here:
We had a poor CMDR who got a brand new ship, and promptly proceeded to lose it. They could not afford the rebuy even with maximum loan applied. The consequence of this is that they lost their ship forever and are stuck in a stock sidewinder again.
If you find yourself in a situation where you don't have enough rebuy, do not undock the ship. Go fly in something else and make money until you have enough to cover at least several rebuys. Sure you may think "I'm not doing combat how can I die" but there are a LOT of ways to die in game, trust me, you do not want to make that kind of gamble ESPECIALLY if you're at a credit level where rebuy is a problem at all.
Now, if this came too late and you're staring somberly at a red rebuy screen, you may be able to