You're fine. Android handles RAM differently than Windows. It's normal, desirable behavior for RAM to run around 75-80% full, maybe even a little higher.
Don't be tempted to use any task killers, RAM memory optimizers/boosters, battery savers/repairers, cache cleaners, antivirus, etc etc. None of that stuff is necessary.
Don't worry be happy. Relax and enjoy simply using your phone.
Android since v1.0. Linux user since 2001.