I'm keeping a secret. 我有一个秘密。
I'll fill you in. 我要把一切都告诉你。
I'll tell you all about it. 我要告诉你一切。
I'll level with you. 我就直说吧。
I'll tell you frankly. 我坦白告诉你。
I'll be frank with you. 我会对你坦白的。
I'll be open and sincere with you. 我会对你开诚布公的。
I'll tell you the truth. 我告诉你真相。