Session fixation is a vulnerability that relies on re-use of a session ID. First, the attacker must be able to force the victim to use a specific session ID by setting a cookie on their client or by already knowing the value of the victim's session ID. Then, when the victim authenticates, the cookies remain the same on the client. Therefore, the attacker knows the session ID and now has access to the victim's session.
This recipe will require some initial reconnaissance performed against the target site to identify how it's performs authentication, for example through data in the POST requests or through basic auth. It will also require a valid user account to authenticate with.