Poser is indeed fun. Making characters and clothing is fun too but a somewhat lengthy learning process. Characters are probably the hardest to make, while props are the easiest. Clothing falls somewhere in the middle. You cannot make a brand new prop inside Poser in that Poser doesn't let you create new shapes, it only lets you use shapes which have already been created. So, to make your first prop, you would need some kind of 3d editing program. Personally, i use StudioMax. I saved for years to buy it and i wish it wasn't so expensive. I am sure there are other cheaper if not free programs out there, but my friends turned me onto StudioMax and i have not looked back.

Basically, here's the steps involved in making a prop Cowboy hat to use in Poser on your favorite character:
1.) Export your model out of Poser as an obj file.
2.) Import your model (in obj format) into your editing program (StudioMax).
3.) Now that your character is inside StudioMax, you can begin shaping the cowboy hat around it.
4.) Create a flat cylinder as the brim. stretch it out to fill the top part of the head. Then stretch the brim.
5.) When your hat is done, delete your model from the stage so all have is the hat.
6.) Export the hat from StudioMax as an obj file.
7.) Open Poser, load the same character you were using.
8.) Choose File > Import in Poser and select the cowboy hat obj file you made.
9.) When you import, the hat should appear directly on your characters head (since you used the model in Max as a guide).
10.) Now all that's left is parenting the hat to your model so it will follow your models head, and finally saving the hat to your library.

I left out all the stuff about textures, also, exporting to and from Poser can be tricky sometimes with scaling and sizes, tweaks are expected, but the above is a pretty rough idea in how to make stuff for Poser.