You can dodge by jamming the stick (movement key) to the side. It pays to keep your distance and wait for them to come to you. Especially when fighting multiple enemies. With multiple enemies, separate them out by backing up and around. Use the right stick to change focus on each enemy.
Your block is holding down L2 (dont know on PC yet sorry). Its got it's own meter on the bottom left that regenerates with some time, if it's down block wont work.
Edit: sorry to answer your Q, I find it best just at attack is coming in. But you can hold it all the time. Best only if you havent had change to dodge.
Train on the horse stance/dummy for endurance, which is your HP. Train attack by sparring at the dojo. And dont forget to eat like Dou Nui every time to make sure your HP is full, the game wont always remind you.
You can also train up your key bound moves (bound to R2 on ps4) both by pressing it waiting for the QTE to appear. Cycle through bound moves with R1/L1 while in battle. Very useful to excute when you've just dodged an attack.
Special
These are my tips thus far. I must say it's not that well explained in game (if you select the advice option in dialogue with the dojo master you'll get a key binding help splash at least.) Wouldve been nice to have some extrapolation on these mechanics through the game I must say.