I’ve been admitted to UT Austin for Undeclared Prepharmacy, does anyone know what the Undeclared means? Is it that I’m not good enough for that major but good enough for the college or what?
Pre-pharm isn’t a major. You have to declare a major while taking the prerequisites to apply to the pharmacy program
And when do I get to choose my major? And does that mean it will be much more expensive? 
You’ll have to talk to an advisor about that. Most people already have their majors when they are accepted into the university. I usually see people majoring in biology or chemistry while taking prereqs for pharm school.
As for how expensive it will be depends on you. I know someone who got into the pharmacy program her sophomore year so she only had to pay 1 years worth of undergrad tuition and then the 4 years of pharmacy school, and many people do all 4 years of undergrad before starting their 4 years of pharm school.