Journia--
I do what all no-talent hacks do... I buy them off the rack (i.e. from sites like Daz3d or Renderosity)!
The two basic ways of making clothes for Poser characters is to; 1) use a modeling program to build the clothing, or 2) make them a part of the character's body mat (skin texture). I've done a little work using the second technique, but have stayed away from the first as I have neither the skills or tools to do it. So I purchase pre-made outfits and costumes for the charaters, which I mix and match to create the look I have in mind. You can also alter the texture mats of the clothes to change the look of the clothing.
In the "Jackpot" render, I used pieces from three different sets I'd purchased to create Alyssa's wardrobe. Her top is from the "Jordan for A3" set (A3 for Aiko 3.0 - the base model I use) available at Daz3d.com, which I chose for its light and airy casual look. The capri pants came from the Capri Fashion for A3 set (also from Daz3d). Alyssa is loosely based on a girl I once worked with who wore only capris pants, so that choice was an easy one. The sneakers came from the Neko A3 Cheerleader Set and helped to complete her look.
I wish I had the tools and the talent to create the clothing for my characters (it'd certainly save me some $$$ if I could), but I'm just happy that there are so many really beautiful clothing sets available for purchase (and some freebies too) to let me exercise my over-active imagination.