Plenty of Task Managers out there...I use Task Panel because it's free without ads. Don't worry about small applications running in the background, they won't slow your experience or kill your battery life. Worry about applications that like to do stuff in the background, like update info from the internet. You don't have to avoid letting them run completely, but you'll learn to recognize which ones do it right (by letting you set intervals), and which ones should NOT be running while you're not using them because they will update in the background when you don't even need the info updated until you open it.
Don't let anyone from this forum try to tell you (and they WILL try) that Task Managers aren't necessary because Linux's memory management somehow stops a poorly coded application from using CPU cycles unnecessarily all the time.