I realize you're a 'newbie' here as we all once were, but surely you have some sense. Asking for "any recommendations" on Barcelona might well take longer to answer than your trip will last. Imagine putting that question about NYC to a New Yorker?!
Barcelona is one of the world's great cities, both historic and modern: there are myriad things for you to do. First, you should be using Google and spending the days in advance of your trip researching and planning. Decide your interests for this first trip - architecture, history, food & wine, beach, art etc - and check the related websites.
True-review has given you some good pointers, but I'd just add to 2 things he suggested:
1. Las Ramblas is interesting but the southern half is very touristy (locals much prefer the much smarter northern part) and there is a lot of street-crime, mainly snatching bags wallets and phones, in that area.
2. Parc Guell is wonderful but capacity and admission are now restricted so you need to buy a timed ticket in advance
Last, I wonder why you haven't bothered to express any thanks to the people who have taken time to respond to your questions.