Welcome to the addiction of RTW travel. My best advice to you is to read the sticky at the top of the forum on how to use the OneWorld RTW product. Download the OneWorld schedule onto your computer. Start listing places you'd like to visit. Are they destinations on OneWorld? Can you get between them on OneWorld?
For me, RTW planning begins with the destinations. Where do I want to go? Where haven't I been before or where is it expensive to go - that I can reach with a RTW ticket? For example, I've been to Easter Island, Dubai, Bali, Victoria Falls, Zimbabwe, and Sapporo, Japan, on different RTW trips.
You've got lots of fun and lots of hard work ahead in putting together a trip. Enjoy!