Help us help you. By posting the year, make, model and engine near the beginning of your help request, followed by the symptoms (no start, high idle, misfire etc.) Along with any prevalent Diagnostic Trouble Codes, aka DTCs, other forum members will be able to help you get to a solution more quickly and easily!
It will be in the line that goes between the Condenser and the Evaporator. There is, usually, a couple dents, or indentures, in the line and a big Union Nut where you can take the line apart. I have seen some, though, that do not come apart. You have to replace a whole line segment. If your AC works, you can FEEL the line. The orifice tube is where the line goes from hot, to cold.
If your vehicle is equipped with a Thermal Expansion Valve (TXV), it won't have an Orifice Tube.
"Knowledge is a weapon. Arm yourself, well, before going to do battle."
"Understanding a question is half an answer." I have learned more by being wrong, than I have by being right.