What is the best class for beginners on team fortress 2

There is not really a "beginners class", all classes are hard to use at first, but in varying amounts of seconds, minutes, hours, days, weeks, months and even years you will master each class. When you first start TF2, make sure you go through ALL the tutorial, it may take a while but it will explain ALL the essentials you need to know to be able to win those games!
It also depends on what team you're on, and what type of game you're playing. One thing I will say is NEVER start with the Engineer, as you need to observe other Engineers first, so you can practice rush building and where to build. If you are on a game like team "Blu" on Payload, it is good play as a Soldier, Demoman or Heavy, however, these classes can also be USELESS on their own, I recommend binding a key to call for a Medic, and having a Medic heal you throughout the game, also, ALWAYS stay with the cart, unless you are playing as something like a buffed Scout for distracting.

Also, make sure you play on your own in single player first and get to know your maps, make sure you know where ALL health packs and resupplies are in each map, if you're low on health and being chased by a Pyro you haven't got a chance unless you can get back up to full health to fight him back immediately!

Hope this helps!