If you are organising a collection directly on behalf of an association or other body, such as asking for donations in the street, you need to make a declaration to the préfecture of the department where it is based and obtain authorisation (if the collection is strictly at local, communal, level, a declaration to the mairie may be enough).
It is possible to be fined for collecting without having made such a declaration.
It should include details of the aim of the collection.
It is also necessary for bodies to keep an annual record of money collected and what it was used for.
If you just wish to make a collection for a charity you support, for example collecting money from friends and family for Ligue contre le cancer in memory of a relative who died from the disease, it is best to contact the charity itself for advice. The Ligue, for example, allows you to set up your own internet fundraising page with the proceeds directly passed on to the charity (see https://goo.gl/BHKPFR).
Further formalities would not usually be necessary.
These charge a small commission for collecting donations made by bank cards and paying the money back by transfer to your chosen bank account.
People usually set up their cagnotte (‘kitty’) on the website and then publicise it to friends on social media.