I'm just looking! 
我只是看看！隨便逛逛！

explanation :  This is a common phrase to use in shopping.  If you enter a store and a salesclerk asks, "Do you need some help?" you might say, "No, I'm just looking."  Another common way to say this is that "I'm just browsing".  This can also mean that you don't want to buy anything right now.   

e.g.  At a car dealer, if the salesman asks if you need help, you can say "No, I'm just looking."