creating OS

Here's a nice thread with a lot of links in it.

A warning: Creating an OS is never simple. You will need a lot of programming skills in to attempt to write your own OS.

so what kind of skills do you need and most importantly what topics are most vital in creating it..i want to know if i can manage to create something very simple..thank you

You need to be an expert in C or Assembly (or C++/Pascal).
A very simple operating system does not exist, so if you plan to go through with this, prepare yourself for the hardest project you've ever made.

i actually want to create one for a PDA because i figured it would need less code and a little simpler than a PC one..i was actually thinking about doing it for a project...i just dont know how i can start and how complex it can get..have u tried designing one before?

operating system design is a specialist discipline that's taught in advanced university courses.
Search Amazon for books about it, you're not going to shake an entire operating system out of your baseball cap in a few days, weeks, or even months.

And don't think a PDA is "simple", it's not.

Creating an OS is never simple. Before you actually begin programming there are lots of theoritical concepts to be learned, for example how will you do memory allocation, how will you avoid deadlocks, resource allocation, processing priority, system failures etc etc. It will take learning of lots of concepts before you actually begin programming.