To be honest, I really think it is possible for you to do Continuers. I didn't do French in year 9 and 10, had no formal French lessons and I'm not a native speaker of French and I got accepted into the Continuers course at the OHS and finding the work OK but I had taught myself French for a year and half before and I had to fight both my German language teacher and my fellow peers to get myself into the course so if you are really passionate about the language and you really enjoy learning it go for it but keep in mind that you do need a basic knowledge of French verbs (if you are planning to teach yourself French, it won't take you that long to learn the fundamental tenses of the most commonly used French verbs though, well except if you want to learn how to use the subjunctive which I don't recommend you learn just yet) and a good understanding of French grammar structure and all that which you will definitely not learn overnight. I recommend that you start teaching yourself now if you really want to do the course but I warn you that you must study the language INTENSIVELY from now on and to start reading French as much as you can. Also be prepared to fight a bit to get into the course.
To answer you questions-
do you think it would be easy to pick up french cont. if you have never done french, ever?
No, unless if you are a child prodigy. The again, keep in mind though that Alexander Downer learned how to speak French in two months (his accent is not that good though. I warn you that picking up the accent is hard as hell and very frustating. INTONATION!)
do you think if someone had enough motivation they could pick it up and do well?
Yes. With steadfast dedication and good and thorough research I believe you can do well.
does it start with a basic thing first and then move into harder stuff or does it assume youve done french in junior school?
No. In my first sets of work my teacher assumed that I already knew-
-the basic French verbs and how to conjugate them (verbs such as 'avoir', 'aller', 'faire')
-basic grammar (how to form a negative sentence) and a knowledge of french syntax
-how to use the conditional
-basic french expressions
-a fair French accent (it doesn't have to be great, mine was so crap at the start of the year, very crap.)
-Knowledge of France
-a good vocabulary. I guess good enough to comprehend most of an article from RFI without referring to a dictionary
RFI - Radio France Internationale, actualité Afrique et monde en français et en langues
and after that it moved onto harder stuff but it is generally revision if you already have at least a fair knowledge and vocabulary of French. I'm guessing that if you've learnt French formally before for at least 10 hours, it will just suffice so do that if you plan to do Continuers.
If you have any questions PM me because I was in the practically same position as you are in now last year.
Bon courage, mec, et souvenez-vous que quand on veut, on peut!