You can start by asking your school librarian. They usually have a great selection of books for this age group. Another way is to check online bookstores. They often have customer reviews that can help you figure out if a book is good or not.
One option is to follow some book blogs or websites dedicated to children's literature. They often feature reviews and recommendations for 5th and 6th grade fiction books. Another idea is to attend book fairs at your school or in your community. There, you can browse through different books and sometimes even meet the authors. You can also look for series that are popular among this age group, as once you find one book in a series you like, you're likely to enjoy the others too.
Well, you could also ask your classmates or friends who are in 5th or 6th grade. They might have some great recommendations based on what they've read and liked. Additionally, local libraries often have reading lists for different age groups, so that can be a useful resource. You can also look for book awards for children's literature, like the Newbery Medal. Books that have won such awards are usually of high quality.