The best you can do is learn from it so it never happen again, it's always a hard blow when stuff like that happens, but is not the end of the world, a lot of big traders were scammed when they were starting.
So in short learn about the most common scamming methods, never accept a trade for something that isn't on the trade offer itself, always double check the items and the values and always be careful with where you are signing in with your steam, if you haven't heard it better safe than sorry and if you did don't go there through a random link someone gave you without double checking the url to make sure it isn't a fake site imitating the original.