Examine your data to determine which data set is the independent variable, which always goes on the x-axis. As Merriam-Webster's Dictionary defines it, the independent variable is "a mathematical variable that is independent of the other variables in an expression or function and whose value determines one or more of the values of the other variables." In other words, if one set of data is months and the other is temperature, months would go on the x-axis since month determines the temperature, not the other way around. Algebraically, the x-axis is always the function, and the y values are always its solutions.
Find the extremes of domain of the x-values, the set of numbers or things the x-values fall within and round to the nearest whole number or 10. This means if your domain ranges from -55.4 to 67, mark -60 on the far left end of the x-axis and 70 on the far right. Put tick marks at evenly spaced intervals on the axis between the extremes.
Determine the best method of numbering the tick marks between the extremes. It is usually best to count off by fives or tens, but if the data set is small enough, or if you have a lot of room to work with on the graph, it is best to go by ones. You may want to consider making your graph neater by only labeling every other tick mark: for example, having a tick mark every five units but only labeling the tens.