Your very first mission or two will be against the AI. When I was in CBT you could only face AI until you were Level 2, after which you could start doing Random Battles (PvP) instead of Co-Op. However, the last update introduced the following for new players:
Introductory Mission
An Introductory Mission will be available for newcomers. The Introductory Mission will help new players learn the game basics, when they log in to the game for the first time.
I just do not know the details on how the introductory mission works for new players, and I don't wish to create a whole new account simply to find out. It will probably be one of the following:
1) Complete the IM to unlock Random Battles
and/or
2) Reach X Level in Co-Op (which doesn't take long) to unlock Random Battles
AI is more aggressive now, that is for sure. Last time I took my CV out, AI CV went for me at beginning of match. It was something they NEVER did prior to patch.