We will have to have some things in common:

http://web.media.mit.edu/~minsky/papers/AlienIntelligence.html

Thus the old but still relevant idea about sending out primary
numbers to get attention and the resulting initial conversations
in mathematics.

The folks who made the Voyager Interstellar Record had to deal
with similar communications issues. In the 1978 book Murmurs of
Earth, they described their selection process for the images placed
on the record.

A photo of a nuclear explosion was rejected on two levels: It
might seem a primitive and feeble demonstration of our technology
to anyone who could find a small probe in interstellar space, and
even then it might be taken as a type of threat.

They also considered an image of a human being embracing the
Milky Way galaxy. While it was meant to say that human beings
want to meet and be friends with other residents of the Milky
Way, it might also have been seen as a desire to take over the
galaxy, so the idea was rejected.

Of course who knows what might offend an alien mind. In Carl
Sagan's book The Cosmic Connection, there is a humorous section
from the old English magazine Punch about how different ETI
might interpret the Pioneer Plaques that were launched into
space several years before the two Voyagers. As one example,
the pulsar map on the plaques, designed to give a general location
of Earth, was considered to be in the rudest position possible by
a race of beings who resembled the pulsar map.

But seriously, as stated above, if they are much too different
from us there will be no real communications. If they know we
exist and attempt to talk with us, this should imply that they
do not have malevolent intentions for humanity (otherwise why
bother to let us know they are there at all, just do a sneak
strike and be done with us) and there should be no intentional
insults or threats in the opening conversations.