Don't worry about it.
Android handles RAM differently than Windows. It's normal, desirable behavior for RAM to run around 75-85% 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. They are counterproductive, waste power, disrupt system processes and degrade performance.
Don't try to micromanage your phone. Don't install software that attempts to either. Let Android handle things as intended. It will automatically optimize operation if you let it.
For more information on how Android handles multitasking, processes, RAM, etc automatically for you read this post by one of the authors of Android:
http://android-developers.blogspot.com/2010/04/multitasking-android-way.html?m=1
Android since v1.0. Linux user since 2001.