If you're new to the role and champion

Just think for a moment. What would be common courtesy? Tell people in the lobby, so they can take the 5 minute penalty instead of a 30 minute 4v5 loss where they win lane then have to handle the Darius/Vayne/Fizz you fed.

Or make a full team of 5 friends who know you're trying something new, so they won't mind and you'll be more willing to accept advice from them.

But don't be the guy who waits the entire pick phase, then lets the game begin and starts by telling everyone he'll probably lose hard. That just makes you look like a punk for wasting everyone's time and thinking you're so much more important that their time is worthless.

And there's really no excuse. Knowing how to play defensive is common knowledge across all roles except jungle. There is no reason you should be aggressive and die due to lack of practice when you know you don't know the champion well enough. Even the best champions can lose that way.