When you start skateboarding, the first thing you should learn is riding, turning and keeping your balance while doing so. (If you don't know where to put your feet on the board, read more articles about stances & foot positions first)
There are a few ways to ride your board. The next chapters will discuss every option.
The most frequently used way to ride a skateboard is pushing.
Leave your front foot on the board, and use your back foot to gain speed by pushing off the ground. You can stear a bit with your front foot.
Always keep your legs sort of together. If you don't, you might end up in a split when you're riding fast.
Some people prefer to push with their front foot, and leave their back foot on the back bolts. This is called pushing mongo and it's not a good thing, because it takes more time to get your feet ready for a trick, so you lose some speed. Try to push normal, if you really can't learn it, there's no shame in pushing mongo, it's just a small disadvantage.
To change direction while riding, you can just lean to either side of the board to go in that direction. The looser your trucks are, the faster you will turn (though you'll lose some stability).
The second way to turn is a kickturn. To do a kickturn, you push a little on the tail with your back foot, and swing the board to the direction you want to go with your front foot while your front wheels are in the air.
Kickturns can also be done on ramps, but you should wait with that untill you're very comfortable on your board.
Tic-tacs are a sequence of small kickturns to alternating sides.
First you make a kickturn to the right, then to the left, then right again, etc...
This might look a bit silly, but it's actually a way to get forward at low speed. Like when your buddy is two board lengths away, you can just tic-tac over to him, instead of pushing.
And of course it's a good way to practise your board stability.
Time to practise the things you just read. Take your time to get comfortable on the board, because it's really important.
Grab your board, cruise around for a while and practise some tic-tacs.