When to start manga?

First of all, you might find this thread interesting:

As for experience with manga – the first one I’ve managed to read was the first volume of はたらく細胞 on Bookwalker.
Speaking of bookwalker, if you have a smartphone, I highly recommend installing a free app for Bookwalker and reading Bookwalker books using it, becaue it’s far more comfortable than using their web interface.

Also, as you are new to manga, you might be interested in this book:

Anyway, best of luck to you – may you read a lot of interesting manga!