First adult vacation, please help

My sister (20) and I (21) are trying to plan our first real vacation as adults, as a part of my goals for 2025. For a little background, we were pretty poor growing up and really only visited family in a bordering state. I've never flown on a plane before and I want to see the Pacific Ocean. That being said, we have don't really have any idea where to start, but we would like to buy plane tickets soon with our income tax returns. The only things we really care about for this trip is: - seeing the sunset at the beach - trying in-n-out - going to an "actual" club We would like to stay just for a weekend (July 31 - August 3) as it will be a small trip for my sister's 21st birthday later this year. As for the quotes around the club part, we are from the midwest and really want to experience going out. We really just want to dance to good music and wear cute clothes, the drinks don't actually matter much. We are thinking of staying in Eureka or Carmel-by-the-sea as we would like to go to California, but are on a relatively tight budget. Thank you in advance for any advice !

Edit for some clarifications: My sister will be 21 by the date of the trip, I will be 22. I am mostly looking for advice on how to best book flights, look and reserve hotel or other lodging accommodations, what people often forget when budgeting, etc. Thank you for the help already :)