ehm ... are you talking about being a webmaster, or creating webapplications?
a lot of what you are asking depend on information you are not giving yet:
are you going to host your website yourself, if not, what technologies will you use (limited by what the host you will take will support).
there are no 'most usefull' languages, they all have a different syntax, but they all 'get the job done'. sure, in some cases, or for some purposes, you'll want to use one language rather than another, but since you don't explain what you try to write, or what it should do, it's a bit difficult to go deeper into that.
simple but up to date ... even a simple html page can be 'up-to-date', it just won't be dynamic. php is a good choice, but gwt can also help you out. JavaScript out of date? wouldn't say that too soon. it's still a handy scripting language, especially if you go into the whole jQuery experience.
as for browsers, I assume there are ways to test for different browsers without actually installing them all, but I wouldn't know one. The good old test-em will still work, though, but starting with FireFox with Firebug installed is a very good choice, it 'll make your debugging a lot easier.