Hey...don't worry, with practice you'll get over it :D
You need a good hook to start with. This hook should be relevant to your topic and provide an entrance to your essay. Let's take this topic;
Some workplaces set age maximums for applicants even though they are far away from the retirement age .Do you agree that every organization has a right to set age maximum for the candidates ?
.... First, get the main idea of it - this one deals with retirement age! So tell something catchy about that;
Retirement age differs form organization to organization. ... this is more relevant to your topic. Now you can start talking about the background of the issue that how some organization act on retirement age.