Random effects are generally used if the levels of the independent variable are thought to be a small subset of all possible values. For example, if were looking at growth rates of a plant on five randomly selected types of soil of all the soil types. In this case you may be just wanting to know if growth rates differed depending on soil.
With fixed effects, you are fixing your independant variable: so that you are specifically interested in certain soils. In this case you chose, or set the types of soil. Now your question might become more specific relating to specific soils.
This is pretty simplistic but I hope it helps.
Was are the variables you are looking at?