Hidsup: not sure how someone saying the person they look for needs to be slim makes you despair? There's no point in them wasting your time if they won't be attracted (that goes for everything they're looking for, not just size).
For online dating you need pretty thick skin unless you're extremely good looking as you'll get a lot of rejections. It might be a little different (as I'm a guy so normally "supposed" to send the messages) for a girl though, just meaning you don't get too many messages.
Make sure pictures are good - ask male friends you can trust, and get them to help pick the pictures - friends won't want to offend by saying you don't look great in a picture if you give them one choice. Try and have a close up profile picture as the main one, then perhaps a picture with some friends, and one with you doing an interest you enjoy. Try and smile, makes a big different to me as a guy. Don't use pictures from previous wedding ceremony I have seen on some profiles before!
Put interests, but keep it short and sweet. Don't have a huge list of "you should be..." or "you shouldn't be..." on the profile, it's just scary!
I think these days most people won't care if they see you on there, more just think "that's interesting she's here". However since it might be people you don't know so well then keep pictures and your text clean, and don't put up too much stuff about yourself. But I don't think it should cause problems to have an open profile.
Just treat it as a bit of fun and expect to mean a few really cool people and a lot of crazy ones too! Then see how things go from there.