lyr = r"path to your dataset"
fldlist = arcpy.ListFields(lyr)
fieldinfo = ""
doc = arcpy.mapping.MapDocument("CURRENT") # Current mxd
df = arcpy.mapping.ListDataFrames(doc) # First dataframe
for fld in fldlist:
fieldinfo = fieldinfo + fld.name + " " + fld.name + " VISIBLE;"
nlyr = arcpy.MakeFeatureLayer_management(lyr,"new layer name","#","#",fieldinfo) # Create the feature layer
arcpy.mapping.AddLayer(df,nlyr) # Add new layer to current mxd
To hide the fields, change "VISIBLE" to "HIDDEN".
You'll need to get the field list, set the fieldInfo, then apply it in makefeaturelayer. You cannot change the fieldInfo properties of a layer that is already in the mxd, so you have to create a new layer with the fieldInfo set to visible.
I am adding this answer because this question hasn't been answered here before. Generally, you should show some more effort in your question, not just "here's my code, please finish it for me." Tell us what you've tried, where you're stuck.