It's been a while since I've played ES1, but AFAIR you should colonize everything you are able to defend, starting with wormhole choke points. That and your ability to colonize planets are two limiting factors on how much you should colonize. After choke points you should prioritize systems that have strategic and luxury resources, and then large systems of 4-6 planets.

As in ES1 you should actually move your existing pop to colonize new system, create colonizers on systems with high food and industry to replenish available stock ASAP.

Score can be seen in tooltip above first top left button when you hover cursor over it. AI in ES1 is rather simplistic and basically it seeks reason to wage war whole game. As such, it is also very spammy on fleets. It is also makes score not a very good indicator of your relative power, as fleet CP are accounted for score. It is better to nave 3-4 super quality mobile fleets that can maw through any foe, then massive fleets of cannon fodder. It is good though to estimate how strong AI is compared to each other to determine who is you main threat.

Just pursue the victory type your faction is best at for now.