If you have water leaking between the grouphead and the portafilter, as signalled by the arrow in the photo, the solution below will help solve this problem.
A leak between the upper boiler and the portafilter is most likely to be a result of the portafilter not being turned quite far enough to achieve a full seal. The portafilter seals at the point where the two handles sit at 90 degrees to each other, or even a little further. The exact position will vary a little between machines.
When the machine is new, it may take some extra force to get the handles to this point; it’s fine to be a little more forceful than normal at this stage. After a few uses the machine will have bedded-in, and it will be much easier to turn the handles to this sealing point.
The photo below shows the point at which the portafilter seals: