Who's your audience and what is your story/premise? That's the best place to start. Are you producing a childrens' style show for adults, or a pre-school age show? Is it an ensemble cast or a few centralized characters? This thought process will help you put a script together (first thing).
Before you write the script, who are you distributing too? If it is local or a normal network, you will have a half hour program (typically 20-22 minutes of show and 8-10 minutes of commercials). If it is an adult swim or online scenario, then you are looking at 12-15 minutes (or even shorter if online). Then write your script: 1 script page in script format equals 1 minute typically. (for more on script format, go here
http://www.oscars.org/nicholl/format.html)
Once you have your script written, cast your actors. Do the voice over stuff first because it is easier to animate to voices than try to match voices to animation.
Then, animate! Depending on the type of animation, it may be a one man show or a 30 person crew. Flash is probably the easiest and quickest animation program (2-D), but there are other programs that are beneficial such as After Effects (2-D and 3-D) and open source 3-D programs. Stop motion and puppetry are also an option, but they require more crew/designers/set up but can be more fufilling for the less computer savvy.
Since your post was dated back in April, you may have already gotten help with this, but if you need more, I have produced animated films and a tv pilot in numerous formats if you have any more questions. Good luck!