to go into more detail about your questions than GJDrissen (nice work tho )
1) Avoid building improvements early on in the game, wait until the planet has around 2 Billion people (Shown as 2B in game) so the colony can support itself and a few improvements without outside sources. i.e. my planet has 2B pop and makes me 25 BC per week (including maintenance) so it can handle maybe a factory, a lab and an entertainment center itself without your homeplanet footing the bills. Also, when you get a high PQ planet (15-higher) with one or two morale bonuses, place a single factory (rush buy) then que entertainment on the bonus morale tiles, then build farm(s), then spam market centers or it's equivalent. An Voila'! you got yourself an economy planet making 500+ BC every turn late game. Another good thing to do (i'm not sure whether or not this is a problem for you) is to avoid building to large an early game fleet, as those military ships need maintenance to stay aloft. Try building fewer and decommisioning when neccesary.
2) Try matching your defenses on the fly (upgrading) to what the latest enemy fleets are using:
Shields < Beams,
Point Defense < Missles
Armour < Mass drivers
Also, use weapons that differ from their defenses:
Beam < armour + PD
Missles < Shields = Armour
Mass drivers < shields + PD
NEVER EVER USE PREFAB SHIPS! always design your own, as custom designs can be better optimized, travel faster (the prefabs don't use engines in my experience), deal more, better damage, and can make the game more personal: ("LOL OMFG MY LANCER FRIGATES JUST P'ZONED UR ANUS...... ROFLMAO")
3) Starbases come in four types (five if you include terror stars) that each have a different purpose, i'll list them in order of value (to me):
#1 MINIG STARBASES- These facilities can be built by moving a constructor over a galactic resource (the colored shapes on the galaxy map) each of which grants you a bonus : Green = economy, Red = military, Yellow = approval, Blue = influence, Purple = Research, and whitish gold is an Ascension crystals (ill get to those). more starbases stacks the bonus, so you can if lucky, have a 500% economy boost or a 300% military with more than one starbase. You may also upgrade your base with more constructors, giving them better minig equipment (increases bonus), weapons and defenses, and miscellaneous things like sensors and ship DeBuffers. Less is certainly not more.
#2 MILITARY STARBASE- These starbases giver a bonus to military ships in the area, and can even give them weapons of a different type that they would not ordinariliy have (in the area of effect). you can abuse this early game by putting a cargo hull loaded to the brim with bukit's o' weppin's into orbit with several well-upgraded starbases directly next to it. this will increase your military ranking almost to the top, if not that. also good for establishing "beacheads" in enemy territory.
#3 ECONOMY STARBASE- Can very, very quickly completely overhaul you economy in under a year. These bases have upgrade modules that can supplement planetary production and reaearch of colonies in the area of effect. They also have modules that are extremely good at increasing trade revenue (10%, 20% and 30% for a total of 60% more income. On Huge maps where a single cluster of routes goes to one planet that may earn over 1000 BC per turn, a single economic starbase can make that 1600 BC! Yet another will give you 2560 BC per turn! it can be a massive boost during economic downturn.
#4 INFLUENCE STARBASE- Very useful only under two circumstances. While pursuing alliances and goodwill towards others, and do not want to be and cannot be attacked by your neighbors. But if the AI manages to steral your juicy class 26 planet (Elysium!!! NNNOOOO!!! give yourself a cookiee if you know where that's from) and you do not want to be immediately attacked while taking a planet. Bombing aliens is always fun, but when you're weak and low on money Influencing a planet with these starbases and their culture modules will let you take over the planet effectively for free, without ships and transports (except 7 or so constructors). Also, you can use Influence bases to ensure your colonies DO NOT fall to enemy culture. Eiether way, you or the computer needs at least at 4.00 advantage on the AI's world/ Your world to overrun it (check this in the planet stats, under INFLUENCE, it should have a number from 0.00 to about 7 or so, this is the reading of influence, assuming it's your enemy planet: Higher = good, lower = whatever)
Special #5 ASCENSION STARBASES- These are technically mining bases only more badass, and you may only build them on Ascension crystals. These crystals make your race ascend to godhood in 1000 turns, 500 if you have two, and so on. You cannot upgrade them, and must guard them well with ships.
Question 4) The planet has ships in orbit, and those enemy ships like to Butt F**K your ships if your not carefu and plan. Check for the "Shield" icon on the lower left of the planet, it will tell wheter it is defended or not. You may also check WHAT is killing you by clicking on the planet and looking at it's starport window.
So there, a noob in need is a friend in need, I hope I helped you! Good luck at GC2!