Just check our own JSTL wiki page for the proper download links and crystal clear installation instructions.
Put your mouse above the [jstl]
tag which you put on the question yourself until a black box shows up and click therein the info link.
Then scroll a bit down to JSTL versions information until you find download link to JSTL 1.2 (or 1.2.1).
Finally just drop exactly that file in webapp's /WEB-INF/lib
.
This way the taglib declaration must not give any errors anymore and the JSTL tags and functions should just work.