Opening speeches are an opportunity to explain your country's policy, introduce your resolution topic and key sub-issues you want the committee to focus on. They are important as they will help you determine the countries you want to work with during the lobbying and merging session.
If you take the floor during formal debate make sure your speech is:
concise
convincing and persuasive
clear
detailed and specific
adresses the delegates and chair in the 3rd person ('honorable delegates..., this delegate is...')
But most importantly, make sure you know the topic at handextremely wellas delegates will ask you very specific questions pertaining to your speech.