Hi Clara,
Good luck with the ebook - what will it be about? If it's not top secret.
As for my advice...
Keep it light and concise. Take the time to edit for brevity, clarity, and tone. As an avid ebook buyer and reader, it always irks me when an ebook is 200 pages long and it could have all been said in 90 or fewer pages. I've often found it takes more than one round of editing to catch the errors (and still, some will slip through).
Keep your audience in mind, their skill levels, and their personal needs. Speak to them instead of talking at them.
Don't underestimate the power of simplicity. Avoid overly decorative graphics and nothing beats good old fashioned black text on white background (this is from experience). Use images sparingly and only as necessary. It doesn't mean you shouldn't have any images, it just means you should ask yourself what the purpose is and whether having it will
add anything to the ebook.
Typography matters. The font you use and the size will aid or detract from readability. Whitespace is also a good thing. I've found that great fonts (especially for printing) are Georgia, Apple Garamond, or Times New Roman at 12-14 pt. Play around with it until you find something comfortable.
Always assume it will be printed and bound. Different people will have different print set ups. That in mind, make sure that your margins will fit the widest range available (basically, have deep margins).
That's all I can think of off the top of my head, and I hope it helps
~ Teli