Hmm, well there's many sites about tips, but I mainly use the one made by the creator's of GIMP. Pretty much because the people who made it, know the most about it. All you have to do is select "Help" on the hotbar, and it will drop down a box saying "Help", "Context of the day", "Tip of the day", etc. If you are brand new, I reccomend using "Tip of the day", and mess around with he tools. Most of the sites are advanced and will leave you behind.
I am pretty advanced at GIMP, or at least I think so. If you need any help, feel free to PM me.