These are explained at the station where they are sold. Highlight the item and wait for the description
Cargo Space Extension 5 units each, price goes up each time. Maximum cargo size is 1000.
Mineral Scanner            21,600   (you'll need it eventually)
Cargo Scanner              43,200   (not very important)
Security License           4,320
You need one for each race. If you kill a Xenon or a Pirate in their space, you'll get a (small) bounty.
Engine Tuning        Available only at Pirate Bases
Rudder Tuning        Available only at Pirate Bases
Like Cargo Bay Extensions, you can pile these on one after another, and the price goes up each time. The STARTING price is around 20,000, and will get up into the millions, so you need to be making money.
Installing Improvements:
Improving your ship is like trading - to buy a Mosquito missile, go to the Space Equipment Dock, select the missile, use the right arrow to pick how many you want, then press Enter.
To upgrade, first select the item you already have and SELL it, then buy the one you want.  The SED is the ONLY place you can remove existing weapons and armor.
(Tip) Sometimes the SED can't sell you what you want, even if you've got the money, because they're out of stock. Try an SED in another sector.