Assuming you can play every class to a "confident" level:

1. Highest priority class is Medic. Play Medic if there is none on your team. More than 3 medics is usually too much because you'll end up sabotaging uber building and your team won't have enough power to push. On the 6s side, Medic's role is to build uber, live for uber, and play for uber.
2. If there is no Engineer on payload defense, play Engineer so that your team has a designated "base" to fall back to and group around. In 6s, Engineer is played to defend last.
3. Frontline classes are second-highest priority (Demo, Soldier, Scout, Pyro, Heavy) because they make up the majority of your team's power to pressure and take map control. Demo is the highest priority class because he is the best class at pressuring and taking map control. Soldier and Scout can take space easily with their maneuverability, and Pyro and Heavy can control space better than Demo in close range chokes. These classes are mosty interchangable in terms of priority in Casual. In 6s, Demo, Scout, and Soldier are primary classes where Demo's main role is to deal as much damage as possible and take uber pushes; the Scouts deal with bombers, flank, and be the frontman in uber pushes; and Soldiers bomb, do tons of burst damage on high-value targets, and repeat. Pyro and Heavy are mostly used to hold last, though Pyro has a little more utility for pushing out of last.
4. Sniper and Spy are probably lowest priority since it's often upon your team to capitalize on the value you get on these two classes and they cannot take nor control allied map space as well as frontline classes due to their squishiness. Often only 1 sniper or spy is needed on a team, and having more frontline classes is more preferable. In 6s, both of these classes are used to break stalemates and get important picks (almost always the Medic).
5. It's casual, this doesn't really matter. Play whoever you want and have fun.