I entered through counter sales. Having a computer/network background, I started working behind the counter in that department. After about 6 months of learning the customer base and products, and helping out in other products, I moved into an Inside Sales spot that opened. I took every training opportunity that came my way (which is easy as manufacturer reps always show up and give free training). From there it's just good customer service and staying on top of the products.
Find out which products have the best growth and profit margin - then hit the manufacturers websites. They usually post training and product information right there. Study it - then when you're comfortable, tell the boss you can handle that product. Presto, you're the expert.