It depends on where on your comfort envelope you are - if it's top of your range one mistake could be disastrous. To get specific advice, you need to tell us about your specific conditions - do you take 1 or more cpu, what level, which map, how do your games play etc.
Big fleet is essential, you need bigger than your opponent. So don't let your ships die, let them run rather. And big is not enough, spamming a single unit can maybe work against cpu but if your fleet is all carriers and LRFs the human opponents will pwn you in no time. Go mixed and balanced - research the other ships and learn what they do. Just building the fleet and putting it there is not enough in the end, however. Micromanagement is important so turn off the autocasting and learn to direct your fleet by yourself.
OK that was the military side (just remember you need lots of military labs for the above). There is a civilian side too and you should never forget that. Upgrade your planets' civilian infrastructure, put up civilian labs, build trade ports, make sure you can harvest a good balanced mix of resources so you don't need to buy and sell on the black market all the time. Build some culture stations. Keep developing both your civilian and military sides, if you fall back in either you will be toast.
Reading the forums helps. But even more help could be getting some advice online. Go to ICO and find somebody with a bit of experience (10+ games maybe?) and ask them to play with you against the cpu (2v2 medium map or something). Ask them for some guidance on what to do and how to beat the cpu - they'll help you win the battle together.
