I'd like not to get into detail about my experience with this kind of thing. I've also worked in areas where you interact with people and lately as a small business owner, which means I know everything about self-promotion. Just kidding. Here goes the advice. Please understand I am not addressing any safety concerns and my advice is not necessarily safe for you to follow when you have stalkers, live in a particularly bad area, or have enemies or friends that make pranks. You need to take those concerns into account on your own (preferably talking to an Internet savvy person before making any hasty conclusions).
Your first and foremost tool in online dating is your profile. Leaving a profile unfinished, for example containing a single photo and a couple of loose keywords, is a mistake because it does not allow people to match with you properly. Do not believe that someone "truly interested" will rise above the scarcity of information and still somehow sense that you are an interesting person. And there is nothing wrong advertising a little.
Photos: You need a good photo to make the impression and deliver the kill. I am talking about making a good impression, not about being seductive (there is a difference). You would probably also benefit from posting a normal, plain picture that shows your everyday look. It will prevent people from expecting you always to look like a model after seeing the "kill" picture. Including several pictures gives a better rounded impression instead of providing only a single angle and particular lights, which may be misleading.
Personal data: Don't lie, ever. Skip if you must but everything you skip may hurt your chances. And if you think skipping certain information will improve your chances, think twice. Sooner or later it will come out. In certain situations you may need to choose an approximate answer or one that best suits you. Also, bear in mind that selectable answers can be used as search criteria in the website's search engine.
Interests, goals, hobbies: Keep it real and engaging. Stick to relevant things. For example, if you went to a nerd convention with your ex, it doesn't mean you're actively interested in Star Trek (which someone may be looking for) and "¡Dos cervezas por favor, rapido!" is not yet fluent Spanish. On the other hand, if you can strip an M72 apart and put it back together under 2 minutes blindfolded or you read Persian poetry, why not tell people. It will give a like-minded person the chance to locate you and it will give the potentially impressed folks a chance to, well, be impressed. Don't skip on something that makes you exceptional out of false modesty. And if you think you're an ordinary person, say it. It takes some guts to do that.
Language: Language is your tool to operate the bigger tool, which is your profile. Use language that can be understood by others. Make sure your train of thought can be followed easily. If you don't care, go play with your dog instead. Don't be ashamed to sound like an educated person if you are. Some people like astrophysicists. On the other hand, don't pretend you're someone you are not.
Limited spaces: If your writing space is limited, stick to what's important and remove information clutter. Think about it as summary writing, which it really is. With practice you can say more and more using less and less room. Even if your space is not limited, remember, however, that people's attention spans generally are.
Moods: It's okay to experience emotions but you don't need to change your profile on that basis. Stick to a general, universal image of you. Don't be negative. If you need to vent, have a beer with your friends. Don't focus on what you don't like, wouldn't stand for, absolutely couldn't accept and so on. On the other hand, don't be false-positive and don't try to please everybody or appear to be a softie when you're not.
Know what you are doing: Verify that your profile sends the right message about you. Always do this. And again.
I hope this helps you in your journey.