At the beginning of the game, you want to focus on beefing up your main weapon, and use that to take out ships. Later in the game, your main weapon will be used almost exclusively for large ships.
After 2-3 upgrades on damage and rate for your main weapon, start working on getting your secondary weapons and repairs in place. Fighter ships are almost worthless, but provide great cannon fodder and distractions. Repair drones are key. You'll want to get 3-4 of them with upgraded repair amounts and faster turnover times.
After those are both secure, buy all of your turrets. I'm partial to the laser turrets, as they have long range, faster fire rate, and when you have 10-15 of them working together, they deal a ton of damage.
By the time you reach level 6 or so, you'll want to start buffing your main weapon again, and giving 1-2 upgrades to each of your turrets. Don't forget to increase your main HP either. Levels 7+ will rain down upon you. You're going to want to max out your repair drones too.
Money cheat: during the game type "cash", "money" or "dough" to get 1000RU - by doing this you won't be able to submit your score
