I havn't played the demo, but here are a few ideas:
1) Learn how to use your tax/spend sliders early on. It makes a big differance.
2) Learn how to buy things (many don't figure this simple concept out, then get frustrated that they are taking so long to build up).
3) DON'T buy on credit/installment. Until you get good with the economy buying on credit can quickly snowball into a major drain on your finances, and ruin your economy.
4) For your first several games, take the initial colony rush slowly, as expanding too quickly can hamper your economy if you don't do it right.
5) The forums are your friend! They are here to help, and if you have a question, go ahead and ask (plus, search around as much has already been discussed in great detail).
6) Don't give up! Just keep experimenting and playing. It took me several weeks before I really got the hang of it, and a couple months before I could compete on higher levels.
Hope this helps, welcome to forums!