Hey - I remember that game. We won (I think) despit you running off to your "stripper girlfriend" (?). All I remember is you saying you needed to pee (?) and then you were gone. In any case, here are the basics I try to play by:
1) There are the essential purchases and then there are the situational purchases. Never buy situational purchases unless you need them because you are probably wasting money that should be spent on essential purchases. Capiche? Easier said than done right? Figuring out which is which and when is one of the biggest parts of this game. This game actually is not difficult to play. It has a fairly low skill ceiling if you are at all familiar with MP RTS. Meaning, once you get it, it's not that difficult. I feel like most competent MP RTS players should be able to achieve the top 90% of the playerbase and then the other 10% is the real pros that play enough to have found those little wedges to separate themselves from the crowd. Anway blah blah blah...on to #2.
2) I'm assuming 5v5 here. First thing is - zoom out - where are you? Are you sitting pretty with two allies safely between you or are you all by your lonesome where no one can you hear scream (meaning two enemies on your flanks)? This determines whether to fleet up quickly with military labs and ships or whether to just colonize, lay down civ labs, and build a nice long trade line with the diplomacy techs to feed resources to your allies who are doing the heavy lifting. A lot of times you will be in between these two states which is more of a balancing act - here scouting is more important so you can know what the enemies are doing. Are they ganging up on you? Leaving you alone? Teching and going for an eco boom themselves? You need to find out. You need to know at all times what it is you should be building or doing. If you are expecting a fight you should have multiple ship factories and at least 30-40 ships asap. Do not build things you do not necessarily need: useless turrets with nothing to shoot at, repair bays at your homeworld when no one is attacking you, etc. Knowing exactly what you need and building it so it's there for you when you need (or researching) is part of the skill. It's like chess -you need to be thinking a couple moves ahead so you aren't caught with your pants down.
3) Your upgrades questions should answer themselves as the game unfolds. There is no set answer. A couple guidelines though are: build up a large fleet (at least 50-70 ships for example) before applying damage, hull, shield upgrades, etc. The upgrades are more useful when applied to a large mass obviously. So fleet supply should be your main focus at first (assuming you are on the offensive). Always get the first two population upgrades on any planet you colonize. Once you have the cash and the freedom to spend it, fully upgrade any and all planets' pop ugrades. The first two will lift you out of a deficit on maintaining them though so they are essential. Which ships to unlock depends on the situation - which is why scouting is again so so important. You should always know what the enemy fleet composition is so you can counter it. Choose your battles wisely. Judging whether you can win battles, or whether you should wait, or retreat is hugely important and comes with experience. Do no just throw ships away uselessly. Always ask yourself before engaging -can I win this? Am I able to reinforce quickly enough if my fleet is smaller to start with? Almost all the ships in the game have a use and a purpose - some are just narrower than others.
4) Advent ships: I usually get 10 corvettes and 4 disciples with a colony cap to start in order to quickly defeat militia and get a couple planets colonized. Then I might get a colony frigate and a smaller group of vettes/disciples to go colonize (back cap) smaller asteroids or weakly defended planets on another flank that is less likely to be attacked. You need to quickly get a sense of where the phase lanes are and where your enemies are - where are the choke points? Where are the most desirable and important planets to grab first? I usually switch over to producing 20 or so defense vessels to counter enemy corvettes next. Probably another 20 corvettes myself as well. Illuminators, sadly, are not that essential to get as they are easy to counter with corvettes. If you build a large enough force of defense vessels though, they could work and are good at taking out weaker capital ships and light frigates. I usually just get one capital and then stall while building a Titan at this point. A really strong push with a larger fleet would take me out here so I need to know whether that's gonna happen (scout). From there - maybe carriers or maybe destra? Guardians are awesome on defense if you are holding a chokepoint - don't get them as part of a mobile strikeforce though as they lose their antimatter with all the jumping and then are useless.
5) If you lose - do you know why you lost? Every game I've lost vs. better players, I've been able to see why I lost. Often I didn't have enough fleet and got hit hard and fast by a larger one - maybe I was fine fighting one but got blindsided by a 2nd player that I didn't see coming. Or I made the wrong decision in where to jump and engaged a losing battle. You should try and figure out why you lost (it could also be nothing to do with you, but poor allies, but there's nothing you can do about that so look to yourself first).
Hope this helps! Remember: Knowledge is Power - each decision you make should be for a reason.
P.S. Once you get a better sense of the game and what to do in various situations, then you will gradually reduce the time needed to think about what to do and just be quickly clicking all your choices and selections. That's the flow you should be aiming for if you are interested in playing competitively online. A faster APM right from the start of an often 1-2 hour game will give you an edge that accumulates in you getting your little space empire up and running ahead of others. The APM requirements of this game are very low compared to most RTS but it's still a factor