How to secure my mail in iOS?
It depends on the level of security you are looking for. For PGP, some options, as explained in this article, are iPGMail and oPenGP. If you are looking for a simpler option, in which you don't want to rely on the PGP web of trust, then you can use S/MIME in the stock mail app with little effort. The linked article explains the easiest way to do this on iOS.