Mc Call's and Simplicity patterns are a good place to start. Both have a "ren" collections and are readily available at your local Joann's or fabric shops.
After you get the swing of it try "Folkware." They specialize in recreation patterns from all periods of time. You should have a good set of skills before going this route as the patterns come with little to no instruction.
Creating your own is not hard its just a matter of trial and error. You'll need some paper, there are pattern papers but I just use brown packaging paper, a french curve for creating neck and arm holes, scotch tape and pencils with erasers. Don't forget the erasers, you will use them. Start by altering currently existing patterns. For instance, make the shirt sleeves bigger or the pants poofier. Maybe add really long cuffs to the bottom of the pants so they can be tucked into boots to create a blooming effect. The French curve is useful for enlarging arm/ neck holes, you will find that many pre-made costume patterns have smaller openings as they were not intended for anything but costuming.
A few other points to help the learning process:
-You get what you pay for with a sewing machine. Fortunately we live in the time of Ebay and most really good sewing machines can be fixed on the cheap. I recommend anything by Husqvarna. Stay away from the computer based stuff until you have aquired a good machine and maybe a serger.
-Your sewing machine probably cannot sew leather. Certainly nothing thick enough to be considered armor. Leather has its own set of tools and skills and its own learning curve.
-Start with basic cottons. Velvets, penne and most other materials can pucker and jump and do all sorts of crazy things that will drive a learning person insane.
-Get a good seam ripper. You WILL make mistakes. Most can be fixed by taking it apart and trying again. And again. And again.
--Chazz
