What's a healthy weight?

I get so confused because there are so many different ranges of what a "healthy weight" is. I understand that I may be at a "healthy weight", that doesn't mean I'm at my optimal weight. I'm 5'3" and weigh 128lbs. This is considered a "healthy weight", but my optimal weight should be more like 120lbs. If I'm considered at a healthy weight already, I've been told that it will be a lot harder for me to get to my optimal weight. So how can I acheive this without dieting like mad for years before I reach my optimal weight?
What can I focus on to reach that optimal weight? How long will it take?