Base it on what you want to do.... do you intend to explore space and colonize quickly? Do you intend to win by diplomacy and culture? War machine? Tech?

Once you narrow down your focus of victory, the path seems clear. However, there are always things you will need.... Sensors to see when an attack is coming. Manufacturing tech for ships, economy tech for mass wealth, and so on and so forth.

In short, there are 2 main tiers to strategy.

1. Macro long term strategy - what is it you want to do (culture flip, research, diplomacy, economics/tourism/trade, fight etc)?
1. There are two main ways to decide what you want to do, big-picture wise
1. Choose a race that intrigues you, look at their abilities to see what they'd naturally be good at (drengin are good at war, terrans at diplomacy, Iridium at commerce, krynn at flipping, etc) and plan on doing what they're good at.
2. Choose what you want to do, then pick the race that does it well (if you like fighting choose drengin, military conquest yor, etc)
2. Set up the galaxy to be conducive to how you want to win - it'll give you a leg up in the game, and an appreciation for your race's special ability (for example, make lots of nebulae if you're playing Iconians, make a large spread out galaxy if you want to turtle, etc).  You can always go back and play again on a "more fair" playing field, but I find that in terms of outright enjoyment, it's nice to reside in a galaxy that supports how you want to play --- it's single player, play the game that you want to be playing!!!  For example, I always turn off Ascension Victory because I personally think it's dumb.  I also turn off tech trading - because I think it's annoying.  I put the game on slow research because it throttles AI that doesn't know how to invest in research properly, etc, etc.  There's a reason that all of these options exist - they make the game a lot more fun.
3. Broadly align your actions with your long-term strategy.  Try to pick an Ideology that offers you the tools that give you a boost to the things that your race is good at.  In general (as a macro strategy) you want to create a long-term structure that expands upon your strengths (because expanding upon you weaknesses will only help you so much, because you'll still suck at them).  If you're wanting to wage continual wars pick stuff in the malevolent tree that gives you unfair war powers, etc.  The same is true for research - try to spend the preponderance of your time researching the things that will make you better at the things you're already good at - pour research into your strengths.  Don't bounce around and dabble, because you'll suck at everything.  If you're making a research-strong empire you can research all the little crap stuff later game when it doesn't cost anything if you want to, if you're making a war empire just ignore all of the rainbows and flowers research - you won't need it anyway...
2. Micro shorter-term situational strategy.
1. This is more subtle and tricky, but in short you will need to shore up weaknesses that present themselves that could cause you to start falling behind.  This can range from everything from picking a weapon tree based upon the defenses your neighbors aren't investing in, to spending money to pay someone to declare war on someone that you feel is starting to over-run you.   It's damage control, putting out small fires so they don't burn down your grand strategy, and also optimizing situational quirks that present themselves in the game.
2. Do not lose track of your long-term strategy - how are you going to win? - don't forget it, if you forget how to win someone else will win first...

All and all it's designed to be engaging and situationally difficult.  No two games will be the same and the path to victory will be curvy, but that's the allure.  If you follow the above advice I think that this will give you perspective on the game, and how you want to play and how you can win.  You will make many, many, many mistakes along the way, and each one is a learning experience (especially the frustrating events that arise).  But I think that this is the outline of an answer to the question that you're asking.  The beauty of the Galciv franchise is that the path to victory has never been a straightfoward one, it's always been about the intricacies of the journey and the roadblocks that the AI puts up along the way.

Hope this helps,

cheers & GL