I'd rephrase the question slightly and ask, 'what problems should I work on?' and 'how can the language(s) I use solve them'. This is one reason why lisp is a compelling choice. You can create your own sub-languages to solve any particular problem.

But for lots of reasons lisp may not be suitable. There are lots of other languages to choose so pick the ones that suit solving different problems. There is not one "universal programming hammer" (though lisp is pretty close) but an array to choose from that may suit different types of problems.

esr in his faqto (faq + howto) recommends python, c/cpp, java, perl and lisp. Each language chosen for their abilities to solve different types of problems ~ http://www.catb.org/~esr/faqs/hacker-howto.html#skills1

Tall order? Take too long? Then read on with Norvig and 'Teach yourself programming in 10 years' ~ http://www.norvig.com/21-days.html