So I have been following the Blender Guru modeling tutorial with minor changes every thing looked OK till he added the sub-surf modifier and I have no idea how to fix (followed tutorial https://www.youtube.com/watch?v=WxMwa0njGSM )
I guess your mesh has something different in the bottom: I think you have a single bottom face (N-gon), which is causing this issue...
if you have this (sorry for the horrible mesh, I'm before breakfast...)
subsurf behaves well
while if you have a single N-gon face
subsurf behaves like in your model
if unsure, share your model through
then edit your question and post the provided link...
edit: after getting your file I found that this model has (still) several issues: at the base there are still n-gons, ruining the subsurf effect, but overall it has some other modeling issues... giving aso bad mirroring results.
About the ngons, this what you have one side
and the opposite is even worse
then below you have some faces with inverted normals
then again, its quite irregular on lines that are supposed to be quite linear instead: this ruins both mirror and subsurf
Nothing terrible, and it can be corrected, but I suggest you to try to model keeping an eye on such things, because it helps you model better, and get better results. First, I recalculated normals (select all in edit mode, then CTRLN)
Then I tried to remove those ngons, getting something like this
if you enable subsurf now, it looks better, but not at its best:
it lacks some subdivision at the base, eg:
with this further subdivision,
the base becomes more defined when you subsurf later
Here is the file a bit corrected (imho, it could still be improved)