You can always try playing spy, but it's very hard staying alive after getting one kill if you're still new. Especially for me, it's hard enough to even
get in enemy lines.
I'm still fairly new to TF2 as well, so I guess we're on the same boat. For me, my very first class was a Sniper, since I didn't know what any of the other classes were and did. Later on, I found Pyro, Engineer, Heavy or Medic as the easiest classes.
Pyro, you don't really have to aim, just in the general direction, since it has a fair amout of AoE. You'll have an advantage over other classes when it comes to burning too, with your suit, you won't catch fire. Just run around holding down the left button will get you many, many kills.
Heavy, call for a medic and fire away in a large crowd of enemies.
Engineer, build dispensers for your team in safe locations, build sentries on control points, or whereever you may need support and in a secure play where Demomen or Soldiers can't easily take down. Teleports are useful for when your teammates respawn and can immediate enter the fray with your teleport.
Medic, I thought would be a pretty hard class, but it's not. Point your crosshair on a friendly class and hold down. Best part is, you don't have to even look at the person you're healing after you've 'locked' on. Just make sure you're within range and walls don't cut off the beam. You'll have UberCharges that can be activated with your right click, you'll know when you have one since the Medic will yell "I'm fully charged!", or you can look at the UberCharge bar on the bottom right. Oh yes, healing damaged teammates will charge it up faster than healing someone who is full health.
I'm not a pro at all, so if I've given any "newbie" advice, feel free to correct me. Just my two cents.
