Keyword Research & Planning
Initial keyword research is the first step. The Google Keyword Planner offers the opportunity to understand search volumes, likely cost and competition for your chosen keywords.
If your chosen keywords are in a very competitive field, they will be expensive, this could eat through a modest budget fairly quickly. Thinking about being more specific or about longer tail keywords may help.
You have relatively little space to get your message across and so ad copy needs to be well targeted, otherwise you can find yourself with a high number of clicks but not enough conversions.
It may be best for you to have a number of highly targeted adverts instead of a single one.