Do you want to focus on MOO1 or MOO2? They're so drastically different it's hard to give general advice on either.
For MOO1, play the klackons on average, medium size, with 3 opponents. It's a good setting to learn on. This gives 48 stars, probably half are colonizable without any planet techs so if you get to 6 you're probably fine. 6 is pretty easy to colonize. Ignore poor planets til last.
It's hard to know what specifically you're failing on so I'll just give general advice. After you colonize a planet and your homeworld is around 70 population move 20 or so onto the new planet. Population growth is a reverse log function so it slows down as you get closer to the cap. I don't remember where ideal growth is but once you get over like 2/3rds capacity you don't grow as fast.
Playing klackons build a lot of factories on your homeworld first before a colony ship. You don't need to max it but I'd probably want about 140 factories before I start construction. This won't take long on klackons, trust me.
For research, determine how much range you need then go all in on propulsion to get it. Usually you can make do with fuel cells 4 or 5 at most. Remember you don't need to go directly to that juicy terran planet 8 parsecs away, you can settle the lesser minimal planet in between that's only 4 away, and then the terran one will only be another 4. Don't forget to send more colonists there!
If some better engine tech shows up get it. It makes your colonists move a lot quicker from one planet to another. But don't research past fuel cells 5 in propulsion, it's too expensive. Being klackon go all construction next and get an improved industry tech, I usually try to get 8. 7 is a little pricey.
After that is where you have to make real decisions. If you've had contact with anyone build a couple fighters/destoyers and orbit planets they can reach so they won't invade you. Then you probably want to research some better missiles and build about 5 bases on your outer planets. I max factories before building bases though. On the races screen make sure you assign some spys to them so you can see their tech. If they get a lot of force fields you need to make sure your missiles can damage them.
Next I'd probably research some planet techs, improved waste disposal is crucial for your economy and you probably want population +20 and then to improve all your planets. Then try for improved robotic controls 3 or 4. After you've maxed your population and factories at 3 or 4 you should have a great production base to conquer. You can now research more weapons, construction armors and maybe some more engine techs and build a fleet to capture the enemy. Again make sure you check info on their techs on the races screen to determine what weapons you need. If they have no planetary shields and only like deflector shields 3 or 4 you can probably just kill them with stinger missiles or something like that. If they have planetary shields 5 though you'll need at least fusion bombs.
Ship building advice, smaller ships pack much more punch cus you can build so much more of them. For the cost of one huge ship with maybe 30 lasers you can usually build 100 small ships with 1 laser. Think about it, 100 lasers vs 30 lasers, the small ones overall are more cost effective. You want to also put the best engines and manure levels you can on your ships and the best computer, even on your bombers because it increases bomb damage too. Make specialized ships carrying one weapon type, either beam weapons for your fighters or bombs for your bombers to hit planets, maybe some missile only ones if you want. You can ignore shields and ecm and never get armor 2, it's too much space, just do the best type armor 1 you can. Having shields and ecm is fine you just probably won't have space. Like it's better to make a small ship without those than have to go up to medium to pack those in. If you don't have enough weapon tech you may have to go up to medium though to fit your best bomb on your bombers though and that's ok, just don't build anything bigger than that.
Now one of the biggest things, you might already know this, how do you open up new technologies for research? The answer is in the manual in the tech section. Each tech group is divided up into tiers. To research something in the next tier you need any one tech from the previous tier. The tiers are divided by lines in the manual. You use this info to skip techs you don't care about and aim for ones you do. For example in computers you want to get improved robotic controls 3 but you aren't sure if it's available. If you own any of the techs from the first group (ecm jammer 1, deep space scanner, battle computer 2) and you can't see robotic controls 3 then you know it's hidden. So now you will have to go for robotic controls 4. If all you're interested in is unlocking that tech, then check you research options for the next tier. Say ecm jammer 2 and battle computer 3 both show up as options. Ecm jammer 2 is a lot cheaper than computer 3 so research that one to jump to the next tier. This information is invaluable and you should consult the manual often.
Another tip that took me a while to figure out, when using your missile bases you can change between scatter pack missles and regular missiles by hitting the button at the bottom of the screen in the middle that says missiles/5 pack. It's a toggle between what missiles are active. Cus sometimes your scatter pack missiles can't penetrate attackers shields and you shouldn't use them.
