How can I securely store my private key I use for logging into my home PC I've been told that the SD card is a terrible place to store secure data because any app can read it.  How else can my phone store my private key I use to log into my home PC ssh server?
I use ssh in a terminal.

$ ls -l .ssh
-rw-------    1 10106    10106          427 Jan  8 07:39 id_rsa
-rw-rw-rw-    1 10106    10106          319 Jan  8 07:39 id_rsa.pub
...
$ pwd
/data/data/com.magicandroidapps.bettertermpro/home


So the private key is protected simply by the unix filesystem and Android's user-per-app design: only Better Terminal Emulator Pro can read id_rsa, and only ssh and other programs I'm aware of run under it.

Whatever your app is, it need only do this to enjoy the same security from other apps.  Full-sdcard encryption, unless it comes packaged with a filesystem having a comparable system of permissions, will not protect your data from other apps.