Ive just bought an iPhone 8plus do I need to fully charge it before use ?

With today's technology, the batteries are much better now. The phones come partially charged and you should be able to use it until it gets down to 1% then charge it as often as you need to for your requirements.

no

Congrats on your purchase. I hope you enjoy your new phone.

Except it's best to not let the battery get lower than 20% and for sure, NEVER let the battery run out completely. Doing so, could shorten the useful life of the battery.