Substituents on cyclohexane can, in principle, occupy either axial or equatorial positions. However, in general, equatorial substituents are more preferred since they are more stable because of reduced steric interactions.

This is shown in the following example for methycyclohexane.
In the equatorial system, the methyl group is anti to the C-C bonds in the rest of the ring system.  However, in the axial conformation, a steric interaction of the methyl group with the two axial hydrogen atoms on the same face of the ring destabilizes the axial conformation. In the images below, this 1,3-diaxial interaction can be highlighted and contrasted with the equatorial conformer. The effect is most obvious when viewed in the spacefilling model.
 

EQUATORIAL	AXIAL
Click here to highlight methyl hydrogen atoms  Click here to reset colors	Click here to highlight methyl hydrogen atoms  Click here to highlight 1,3-diaxial hydrogen atoms  Click here to reset colors

The larger the alkyl substituent is, the greater the preference for the equatorial position since the larger the group the greater the steric interaction with the axial hydrogens.