Welcome to the game!
I'm fairly new myself, so I have just a few tips.
First of all, if things are obviously going wrong, the game is trying to tell you something. It's a 'learning experience'.
Learn from what it's showing you! Figure out what went wrong, and do it different next time.
In anything but the smallest galaxies, it's important to 'colony rush'. You've got to get the good planets before the computer does. A quality 26 world with a bunch of juicy specials like research, approval and manufacturing bonuses could be yours, or it could be the computers... it's whoever gets there first.
So figure out how to get a lot of colonies started. That's rule number one.
Second, you've got to get your colonies happy, because if they aren't happy, they will 'flip' over to the other civilizations, or won't get good population numbers quickly. I try to colonize, and then build in this order: Factory (hopefully on a 100% bonus manufacturing tile, NOT the 700% bonus tile... be careful of that one, it'll suck your money dry!). I build the factory first, then a 'morale' building (Entertainment Network), then, if there is room, Factory, Factory, Spaceport. On Mars I build Factory, Entertainment Network, Spaceport, and I'm done.
Most every planet gets 3 factories and an entertainment network (morale building) started building as soon as I colonize.
One reason you might have fallen behind the computer is the SPENDING slider... It's under your Domestic Stats screen. At the top is Taxation, then under that is Spending. As of this version, it sits at 66%... you are only making things 2/3 as fast as you could if it's left at it's default position. If it doesn't run your economy into the red, keep that at 100%!!!
So, get as many colonies as possible. Scoop up the choice worlds before the computer gets them! Then get them off to a good start by getting some manufacturing ability (3 factories) and keeping good morale numbers (by getting an Entertainment Network building).
Once you've got that as a base, learn something from every game you lose.
In the last few games, I learned:
Don't neglect logistics! The computer attacked me early, he could build fleets of small ships and I couldn't... I lost even though I had equal weapons tech to his because he got bigger swarms of small ships... of course it was mostly because I hadn't gotten enough colonies started, and his economy was so strong that he was pumping out tons of those little ships...
Trade technologies for profit! I usually research rush to get better engines so that my colony ships can get more colonies than the computer players. Then once I have colonized everything, I trade the propulsion technologies to every other race. Trade for other technologies, and then ask for money on top of what you are trading... or trade just for money. I don't care how fast their ships go once I've scooped up all the good worlds, and the money helps me jump start my new empire. This really shows the value of the diplomacy techs, because you can sell off lower versions of what you are researching for a lot more if your diplomacy is good.
Trade routes! The computer showed me this... build as many trade ships as early as possible and get them out there making money for you. Trade frigates are worth the cost/time of building them early.
The computer also showed me to build Economic Capitols as early as possible. Money helps.
Keep playing. You are going to lose some games. But if you keep learning from your defeats, you'll get better and better. It won't be long until you are winning regularly.
Good luck!