I'm not in the start-up world personally but from I understand it's all rather vibrant. I went to Vivatech and was, despite the lack of air in the building, quite impressed with the sheer number of start-ups present.

Having French language skills is a HUGE benefit and was not too long ago an absolutely necessity. It's getting better now and you can more or less get by speaking only English, at least in a professional context. Socially it won't be that great, although if you're motivated I'm sure you'll learn enough for everyday conversations rather quickly.

A good website to check out job offers is https://welcometothejungle.co (in French only unfortunately). LinkedIn is big (how I got my previous job). Also, Hired I've heared (no personal experience).