Trade at the start, buy low, sell high. Energy cells should start you off fine, though it's alot less lucrative in X3 than in X2. After which you wanna be aiming for stuff like Silicon. Earn enough and buy a basic trading ship (TS), Argon Mercury is fine for starters, though I personally go for the Caiman Super Freighter eventually. The TS ships are high capacity ships that let u load a crapload of cargo to sell, so that's your best bet to earn some good money to buy your better ships and build your first station. Do whatever missions you see in the bulletin board that u think you can do to get that extra money and reputation. Get if you're in the hundreds of thousands of credits, get a jumpdrive ASAP, buy navigational satellites and load them in your ship, then explore each sector and put the satellites in whatever sector you think you'll be actively trading in or buying ships from. When you near the million mark, start to setup Sector and Universe Traders which will do the trading for you and earn shitloads of cash. Buy a starting M3 fighter when u hit the millions for the storyline, killing Xenon and combat quests. And so forth

The rank and class of ships as follows, at least in X3: Reunion, more classes in X3:TC and AP.
TP - Passenger Transport, think taxi missions
TS - Trade Transporter
TL - Station Transporter which you need to either buy or hire to build your stations and economical empire.
M5 - Fastest flying scout ship
M4 - Interceptor class fighter, decent speed, + for heavier version
M3 - Your normal fighter, + for heavier version
M6 - Corvette Class (Think Corellian Corvette in Star Wars equivalent)
M7 - Frigate
M2 - Battleship/Destroyer capital class. With the most guns, heavy shielding and hull.
M1 - Carrier capital class, your standard carrier for deploying mass fighters in the shortest amount of time.

Endgame, if there was ever one, consists of you having a massive trading empire with your factories, stations and traders all over sectors and you earning money automatically as time passes. And fights involving flying your M1 Carrier and M2 Battleship or more, side-by-side, warping to the enemy and unloading about 40-60 M3 fighters from the Carrier to engage the enemy, while you either watch the fireworks from the bridge or hop onto your fighter and engage the enemy from your Carrier yourself. It's deeply satisfying and just simply breathtaking experience.