I met my soulmate on Plenty of Fish. I think if you know what you're looking for and won't compromise, you can. However, I wouldn't necessarily expect to find a lifelong partner on Tinder, for example. Research the sites, ask what people think of them. Look for ones, like Match and eHarmony, that have a reputation for forming real relationships.
Also keep in mind that while you can find a good match on a free site (I didn't pay and found the man of my dreams) , you may find better quality if you both have to pay for the site. A financial investment tends to make people more serious and clear on what they want, and less likely to play games because they don't want to waste their money.
Be honest as well. If you're looking for commitment, say so. Don't pretend to be open to things that aren't what you want. The right one for you won't be scared off by knowing what you really want — after all, if they're right for you, they want the same thing you do.