The first hundred turns, and your decisions therein, really make or break your game. It decides how you must play for the rest of the game.
In my games (mostly default settings, challenging difficulty, small maps with 4 opponents), the first decision I have to make is if I should colonize that small planet that starts next door to me... or if I should send the colonizer to a distant star in a gamble that it (a) isn't an enemy controlled star already, or (

won't be colonized first by a similarly enterprising AI. I can say 50% of the time the AI beats me to a primo planet by 1 turn, so the gamble is usually worth taking.
The next decision you have to make is if you want to rush attack any nearby opponents. Many will start declaring war right away and launching ships to the nearest planets, mopping up stray fighters and keeping the board "clean" while waiting for your researchers to get to planetary invasion.
Or perhaps you will instead want to focus on starbase construction! Defensive/offensive starbase techs can make your puny little fighters able to fend of the largest fleets for a loooong time, giving you time to expand in other areas. My favorite is the suprise-popup attack where I suddenly spring a huge fleet using techs I've researched for hundreds of turns, vaulting my military strength from lowest to double- or triple- the competition.
Then there's always the influence play! You can dedicate a lot of resources to influence (usually paired with trade and diplomacy), and not even piss off the competition along the way (if you use influence starbases near your planets instead of the enemy's). Honestly, I start most of my games assuming an influence ability but in most cases it doesn't turn out that way and I have to change course halfway through.
Then there's espionage! I played a game with a spy-heavy race, where I kept two civilizations completely locked down. One spy on each tile of their two planets, small fighters shooting down the occasional colony ship or asteroid miner they could produce. I kept this pressure on for hundreds of turns until I had an invasion fleet researched and built, then I took over fairly easily. My spies were then returned to my control and I could re-deploy them to the next poor suckers!
And, as always, there's the all-out good-planet strategy. Build up your planets into superstructures of amazing technology while maintaining a so-so military (just big enough to discourage war). Don't focus on expansion whatsoever, just play to be a good neighbor!
After my hundreds of games using the above strategies only recently did I load up a truly evil race and begin playing by researching exclusively offensive technologies. And once again, I've changed up my whole starting position.
There are so many ways to play out the first hundred turns you can really change how the game plays out. I think your path up the research tree mainly defines it. Are you going for space weapons? starbase offense/defense? influence techs? politics? trade/economics?
Are you bored with your routine? Try banning yourself from something. For example, ban researching offensive weapons and see how you do. Try banning all research up the planetary-improvements tree (no fancy factories, no fancy banks, etc.) You'll be suprised how you can adapt!