How do I convince my step-mom to not force me to wear tighty whitie briefs after I've worn boxers for 2 years?

Tell her she is making you uncomfortable by telling you what kind of underwear you can and can't wear. At 14, you are plenty old enough to pick out your own underwear and the fact that your step-mom is seeing your underwear or even thinks she should be able to tell you what kind to wear is pretty creepy. Tell her that and see if she backs off. Tell her to knock before entering your room too, your step mom shouldn't be looking at you half-naked at age 14.