A cast-on is the first thing you have to do before the real knitting starts. It is the technique that puts the yarn on your needle.

How to get started. First of all you need to know how many stitches you have to start with. Then you estimate the amount of yarn that will be needed for your cast-on. You can try to wrap yarn around your arm and count the stitches you are able to cast on. That makes it much easier if you need to estimate the length again, because the thread was too short or far too long.

You wrap the yarn around your thumb, take it on your needle by slipping the needle through bottom to top. Now you take thumb and index finger of your left hand and let one thread run in front of your thumb and the other behind your index finger. You use the remaining three fingers of your left hand to guide and hold the rest of the threads towards the back. If you now move the needle downwards, you'll see that the threads cross at the base of your thumb. Insert the right needle into the loop, created by your thumb and slip that loop of. Pick up the thread with your thumb again (and keep holding onto that thread around your index finger), insert the right needle, slip. Repeat until you have enough stitches to start with.