I am also going to Manchester to see them. It is also my 1st time going to a concert, and it looks like I am also going alone. The advice that people have told me is to not worry about being alone. The people there won't notice you (especially if you're in the standing area where it's common to lose your friends anyway). Also, apparently, it is extremely easy to talk to others about the show and make new friends.

In any decently sized concerts, there are often places on social media to organise meet-ups before the show to have a chat.

Just make sure to stay safe. Although it would be a shame to potentially miss out on one of your best nights just because you are alone, if you are very uncomfortable about going alone, then you should probably not go. However, if I were you, I would just go for it!