Is something that I can do at the time of posting something to my blog?
Not unless that's a specific feature that Polyvore offers. I checked their FAQ and don't see that mentioned as an option, but if you click the "Still have a question?" link at the bottom left, you could contact their support team directly to find out for sure.
What I meant is that after you share something on Polyvore to your blog, you could go into your site and assign a category to that post - for example, some categories could be shoes, dresses, makeup, skirts, etc.
To start to get more comfortable with WordPress, you may want to check out our guide for beginners: http://learn.wordpress.com/
In particular, you might want to peek at the info about posts vs. pages: http://learn.wordpress.com/get-published/
Let me know if you need any other help!