try a medium random map with four (or less) players all set to easy.  maybe give yourself an ally if you need it.  turn pirates off.  and either play entrenchment or the original sins of a solar empire version instead of the diplomacy.  this will help limit the features you need to learn to defeat the ai and give you more time to build fleet and turtle into your planets.

YES!  YOU ARE REALLY BAD!

I'm joking.   I consider myself pretty good at RTS games, and sometimes I even get by butt kicked.  (I'm a sore loser to AI too.  Like they take one of my major systems and I rage quit.  don't have that problem against human opponents, but those AI players have no sense of humor)

Yeah, what the the previous posters said. what difficulty are you playing?

As with any RTS, maximizing income vs. outcome.  If you ever have enough resources to build something, do it unless you're saving for something specific.  (in which case you should get THAT the second your resources meet it)

Are you being sure to purchase the population upgrade for the planets you're taking?  If not you'll end up having little credit income.  It's a common mistake made by many beginners.  Also, try to think, "fleet, fleet, fleet!".  Try to build a large fleet of long range frigates.

From the sounds of it, you weren't expanding at all and were getting beaten by an AI which controlled 90% of the solar system.  You can double your income in the first five minutes of the game if you expand aggressively, and a an experienced player can easily top 100 credits per second within the first hour of the game if the enemy is playing passively.  To put it simply, you will get crushed by a much larger and more powerful opponent if you don't grow your empire.  You don't suck, you just don't know what you're supposed to be doing. 

When the game begins, almost every planet is controlled by neutral militia.  These are not allied with the enemies, and they will attack them as well.  You will need to destroy these militia to capture the planet.  It's very important to scout planets early so you know how many units are defending and how many you need to build to capture them.  Generally your capital ship can handle all but the most powerful of militias. 

the population upgrade is critical.  without it you will be taxed into nothing as you expand.

tech upgrades to increase resourecs and income are also helpful.  one error i have seen several people make is that they increase the fleet limit way too early.  even if you arent using the capacity, the tax is still in effect.  against the ai, only upgrade the fleet capacity after you have finish getting your income in order.

with all settings normal, you can build a colonizer capital ship and a few frigates right at the start.  use these conservatively until you capture a few planets/astroids.  focus on your income after that keeping fleet maintenance taxes low.

everyone will have their own suggestions and techniques, but follow these basic practices and you will have fun and rule the map.

There are some very good replays of pro players how to develop economy. Those are most in bigger matches 3v3 - 5v5 but if you watch them I'm sure you will get some idea how it is done properly.

Of course you will have to be slower since you are building fleet as well but one advice especially against ai is to go full economy (don't purchase any additional fleet supply and sell your cap ship factory as soon as you build that first free cap) and spamm trade ports on all planets ( leave 3 slots empty so you can build military labs a little later on ) and on planet 1 or 2 phase jumps away from enemy build 2 or 3 frigate factories. Sell factory on your HW and build another trade port. And now you can go fully military do research and build ships..... 

PS Don't have time at the moment but if you want i can post some of my replies. I'm not the best in sins but i'm good enough...