Is it possible to mettre en ligne a story using Pages? The only other format I have is templates and I use a Mac so my options are limited! HELP!!!

No it's not possible. I know, it really sucks, I have that problem too. But what toi can do is copy-and-paste the story into a blank Google document, and then click file, download as, and it will give toi different options, many of which are compatible with fanfiction.net. I personally use ODT format, just because I feel like it, but I think RTF works too.