It's my first time trying to make a car on blender. I have created half of the object and then I have duplicated it. I used ctrl m to mirror it in the y axis. What would the best way to merge the two halves? I have tried to merge the vertices and just adding faces to the gaps but it becomes deformed because of the subsurface division modifier.
I think you should try turning on 'Clipping' in the mirror modifiers tab and then try moving the vertices closer to the axis. This might probably help https://youtu.be/WxMwa0njGSM?t=1218
What you can try is take an empty axis in the middle of your end of the 1st half of the car and mirror it accross that axis using a mirror modifier. (Axis not to be in middle of whole body but at the middle of edge where your car is cut half). don't forget to select the 2 axis across which the car would make sense.