You mean like:

1
2
3
4
5
6....

If so, just type the first few, and then click the little black square, bottom right of the highlighted cell, and drag down to the desired number

 

There is also the "Edit-Fill-Series" function which will work for dates and other formats.

 

for a simple task like this, the above solution is easiest - but you should know how to use excel properly to get the most out of it

excel formulas will allow you to do more complex things but also simple things like this:

in cell a1 you write: 1
in cell 2 you write: =a1+1
you can then drag cell 2 all the way down

 

And dont forget to copy cells and paste special as values.

 

Well, if you want the numbers to adjust if you insert or delete rows then you might want to try (example formula in cell A4):
=ROW(B4)