Catering event for 1st time. How should I prepare?
The most important thing is to check everything and then check it again. 

Test your recipes and figure out what can be done in advance without reducing the quality.

You should have checklists of everything that needs to come with you (food, cooking utensils, serving utensils, eating utensils, decoration, cleanup gear, etc.) 

You must pre-scout the location and make sure you have thought through how your logistics are going to work - do you have the refrigerator and burner space you need, for example? 

Think through how everything will be transported as well. Will you be able to keep everything at safe temperatures? Make sure all containers are thoroughly sealed, and if there is any risk of spills, add a layer of masking tape.

Bring extras of everything. Inevitably, something is going to spill or there will be more guests than anticipated.

Then check everything again.

Also, keep good records of what you made and what got eaten so you'll be ready to do it next time.