Scratch is a graphical programming language. It is great to learn the logic behind programming such as loops, if statments etc. It also teaches you about more advanced things like objects too. It makes it really easy to make games and graphical programs, but it is slower then written programming languages, and can't really be used outside of scratch (though Bert does know a way...) Find scratch here.
If you like the idea of scratch, you could also try appinventor which lets you make android applications, or have a look at this webpage. There are even some 3d versions!
Python is a great language to learn. It's on the computer lab computers, is really easy and pretty much reads like english. You don't have to bother about compiling or anything else, just double click the file to run it. You can even turn them into executable files that don't require python installed, so they will run on any computer. It's fast, but also really easy and fast to program something in too. This is our recommended thing to start with, and you can find tutorials at codeacademy.
HTML is used in every webpage. It is necessary to learn if you want to make your own website. You can learn it easily, and can keep learning new things all the time as you go into more complicated web development. It's easy and fun!
CSS is always used with html, and it changes the website from a not nice looking webpage to a nice looking one.
HTML is the structure, and css is the presentation. So it's not suprising these go hand in hand. But CSS is also really easy to learn and you can quickly set up a really nice website for yourself. If you're a very creative person and like things looking nice and know when things don't look nice, css is definitely something you'll enjoy. If not, it's still really important, though you might not use it quite as much as those people.
You can learn html and css at codeacademy or have a look around the web for another tutorial. once you've picked up the basics, w3schools is a great website if you've had enough of tutorials and want to make your own website. it makes it easier to find exactly what you need to make something that colour, or this big, or whatever else.
Codeacademy has a variety of other languages not talked about here, such as java. You can try one of those if you wish. Or have a look at some of the advanced stuff: perhaps there's something in there that you think you should do. If you know what you want to do but it's not on here, that's okay. As long as it's coding it's fine.
C/C++/C# are all more complex languages to learn, though they make up for it in speed. C is the oldest and not object oriented which makes it way harder to do bigger programs. C++ is, and it is the best language of the three in my opinion, and the most used. (unless you are doing unity, see below.) C# is a even easier language, but has a lot of disadvantages, especially in speed and compatability, as c# is mainly windows based. You can find plenty of tutorials online for all of these.