As you have ideas, you'll find that they start to fall into natural groups. Mindmapping can help these groups of ideas show up, as can asking yourself whether a number of ideas are all related to a key issue which can be summed up by a key word. You can also find it useful to group ideas on the basis of what kind of cues they send to your reader - some ideas belong more naturally in the introduction or the conclusion, for example.
Organizing your ideas is about working out how these groups of ideas relate to each other. Talking the essay over helps with this, since as you explain things to someone else, you are having to organize ideas in some way. It can also help to look at how the topic you want to write about is set out in books or articles you have taken notes on. You will also want to move from mindmapping to the more structured format of outlining at this point.
You won't want more than about three or four main groups of ideas, since each group will correspond to a section of your essay. If you have more than four main groups of ideas, then you'll have to decide which group to cut - deciding to leave things out can, paradoxically, be a very productive step in writing your essay, as it frees you up to devote more attention to the essay's rhetorical situation.
Feeling you have to include all your ideas in your essay can lead to a very writer-orientated mode of writing which doesn't consider issues of design for a reader. A good rule of thumb is to aim to cut out about a third of the ideas you've come up with. Ideas that you've decided to leave out can still play a suprisingly important role in the essay, since an important aspect of writing is working out what can be left to the inferences of your reader.