1: The easiest way to sign up to rides is using the Companion app.
Find the ride you want to do and tap the [+] button beside the category.
You’ll be prompted to get a reminder.
Some minutes before the ride, start Zwift (the game). You should find a button appear asking whether you want to join the ride once you’ve actually started riding. You won’t see it on the world choice screen.
If your ride isn’t one of the three worlds for the day, don’t worry. Join any of the available ones and the game will take you to the correct world.
2: If you’re doing a working, you’re usually in “erg” mode. In erg mode your trainer sets the resistance, and it doesn’t simulate gradients as such. So you can feel a high resistance on the flat, or little resistance even though the road is going uphill.
If the resistance is too high to pedal against, it’s probably because you’re not pedalling fast enough. In erg mode the power is the target, and power is torque x RPM (this may not strictly be true, but it’ll do for illustration). Torque is the force needed to rotate a mass around a given point (roughly, as I understand it - I’m not an engineer). So the faster you pedal, the lower the torque needed to reach the power. If you’re pedalling slowly, the torque is higher.
If you’re pedalling too slowly, the torque required to hit the target power can be too much to even turn the cranks.
You need to get on top of the gear as it increases, and reacting too slowly leads to a spiral of death - where your cadence is too low, but the game wants more power, so it increases the torque. That makes it even harder to pedal, so your cadence drops and the game increases the torque again to try and hit the power. Before you know it, you’ve ground to a halt.
Best tip is when you know a higher power interval is coming up, you increase cadence a second or two before the power increases.