For missions, if you have not done so, make sure your cargo racks are full-size. The cargo racks that come with the starter Sidewinder are half-sized, so replace those and your profit from cargo will instantly double.
Early on in the game it is important (because of the Sidewinder's limited jump capability) to make sure the combined mass of ship, cargo and fuel still allows you to travel to your destination before accepting the mission, using the cargo mass slider in the Galaxy map. It is a good idea to refuel your ship first when you dock because the mission board will not offer missions you cannot complete in your ship's current state. The problem is: when you accept a mission while low on fuel and then fuel your ship, the increased mass of the fuel added to your ship after the mission was accepted can render your ship incapable of making the jump to your destination.
I used the "Planet Express Ship" approach to the game, making deliveries to the local systems, using my starting-point station as a base so as to build local faction REP, which leads to more lucrative missions, which leads to moving up in the game, more credits and bigger/better ships. On the subject of bigger, better ships try to resist the urge to buy a new ship just because you can afford the hull, that is a huge credit-sink for new players. Keep upgrading your Sidewinder until you can identify a definite need for (more cargo capacity, more jump range, more weapons, etc.) a different ship. The first thing you should be upgrading on your Sidewinder (after the cargo-racks) is the Frame Shift Drive (FSD). This will give your ship the legs it needs to get you "off the porch" and "out into the neighborhood". Once you get that far you will probably have a handle on the basics and be just as lost as the rest of the long-time players. Welcome!
PS: The Sidewinder is a great ship and can do everything in the game on a micro-scale. Don't sell it! Resist the urge to "trade it in". Initially, try and work your way into a Cobra III, the most common, capable and flexible "small" ship in the game. o7