For example, the skeletal formula of hexane (top) is shown below. The carbon atom labeled C appears to have only one bond, so there must also be three hydrogens bonded to it, in order to make its total number of bonds four.  The carbon atom labelled C has two bonds to other carbons and is therefore bonded to two hydrogen atoms as well. A Lewis structure (middle) and ball-and-stick model (bottom) of the actual molecular structure of hexane, as determined by X-ray crystallography, are shown for comparison.
It does not matter which end of the chain one starts numbering from, as long as consistency is maintained when drawing diagrams. The condensed formula or the IUPAC name will confirm the orientation. Some molecules will become familiar regardless of the orientation.