Choose your graph scales to make plotting and interpolation simple and to spread the experimental points out over most of the graph page. Label the axes and give units for the plotted variables. Errors on graphed points should be shown as error bars, in both directions if necessary. Finally, ask yourself if your calculated results are reasonable. Check your arithmetic by doing calculations with rounded off numbers.
Systematic errors affect the accuracy, random the precision of measurements.
The calculation of the error in the final result of an experiment is not just an exercise in arithmetic, it is necessary in order to make a scientific statement about the reliability of the result. Remember, however, that it is better to make a realistic estimate of the errors, rather than to get lost in the mathematical formalism. The overall precision of a measured quantity does not only depend on the uncertainty in reading a scale, other factors, e.g. a personal decision as to when a pointer is first seen to move, may be involved. If the number of readings of the measured quantity is small, or the spread of readings is not random but is determined by e.g. the limitations of the measuring instrument, the traditional estimate of error is the 'maximum likely error' and error propagation involves summing differentials. A transparent ruler helps here.