A few years ago this would be a fairly easy question to answer, but these days it's not quick as easy to provide this type of advise. It's also not very easy to provide advise without knowing your situtation...that is, what is available to you at your location and what your current employeer allows and will not allow in regards to programming languages.
The obvious answer is to learn whatever high level language your company is currently using. Probably RPG or maybe Cobol, but you also need to know what languages are available there in order to determine what opportunities are avialable to you. What is their primary HLL? Do they have other compilers available, such as C? Do they have Zend PHP server?
After finding out what languages you can use on your employeer's System i you could then do a little research to see which language is in the highest demand today. Some languages, like RPG, may limit you to this platform while others may provide opportunities on other platforms.
I think if I were just getting starting on the System i (and knowing what I know about it's decline over the years) that I would focus my attention on things that do not limit me to the System i. I would learn SQL (including stored procedures), HTML, Java!--script-- (not Java), and PHP. I would perhaps even learn CL, but would avoid learning any HLL for the first couple of years (with the exception of perhaps C).
After a couple of years I would start this process again to see if I wanted to learn a HLL for the System i or for some other platform. This decision would probably be based on where the best and most opportunities are.
If you are determined to learn an HLL for the System i and don't really care about making yourself more marketable in a generic sense then I would still start with SQL, HTML, PHP, and also begin learning RPG. But I would also be sure that I am ready to relocate to any part of the country in the event that I loose my job ;)