I start Year 9 next year. When I was in Year 7 I did French and came first. I didn't find it particularly difficult (my first language being English).
French and German are obviously completely different languages. Learning one first does not necessarily give you a head start in the other. In German, words are easier to pronounce. German words are pronounced how they look (letters have set sounds with few exceptions), so you might find it easier to pronounce German than to pronounce French (French words are not always pronounced how they look).
It is said that German is an easy language for English speakers to learn, because English and German are both Germanic languages. But then, again, I didn't have major troubles learning Beginners French in Year 7.
I would say that you should learn French first, as it is more widely spoken and you could then use your knowledge of French to help you learn other Romance languages such as Italian, Portuguese and Spanish. In the end, it's up to you.
As long as your are motivated, patient and continually practising whatever language it is that you choose to learn, you will eventually succeed.