One way is that you can both find the same public server and join it. Then become allies and find each other on the map.
Another way is for him to do whatever new user qualifications he needs to do to be able to add you as a friend on Steam and then either you or him start a game and the other join by selecting "Join game..." from the friend icon menu dropdown.
If neither of you have extremely powerful machines then I suggest the first option. Plus, you don't have to wait to be friends on Steam to do it.
Another way is for him to do whatever new user qualifications he needs to do to be able to add you as a friend on Steam and then either you or him start a game and the other join by selecting "Join game..." from the friend icon menu dropdown.
If neither of you have extremely powerful machines then I suggest the first option. Plus, you don't have to wait to be friends on Steam to do it.