The command you need is IF.
The basic format is =IF(AB=5,X,Y)
Translated this means IF Cell AB = 5 then Cell that this formula is in will be value X else it will be value Y.
To do want you want all you do is nest IF statements
so you would put in a cell where you want the answer
=IF(AB<1,AB*2,IF(AB<5,AB*1.5,IF(AB<50,AB*1.3,AB*1.2)))
Just be careful with the Brackets, as it's dead easy to get these mixed up, and always work in value order.