If your parents have the personal finances, would recommend a pair of khakis. Could you borrow from a friend? If not, a pair of slacks or jeans would work. If you wear jeans make sure they fit well (not falling off, but not skinny jeans either), clean and no holes. Check the weather and wear a shirt/jacket appropriate for the weather. I remember plenty of kids coming in jeans, button downs (tucked in), belt and loafers or deck shoes and they looked just fine. Part of this process is interviewing along the way, so an investment in khakis, slacks, and a nice shirt for congressional interviews, meet and greats, BGO interviews, college nights, etc are probably good investments. Shop thrift stores, sales, etc. You can look good, without spending a ton.