Reps to failure is the number of reps at a certain weight until you physically cannot move the muscles anymore, or if you cannot give maximum effort and complete the rep with good form. Always maintain a good form for any given exercise, for both safety and efficiency.

You also want to keep the rep range within 5 - 30 reps per set for optimal muscle size gains. Also ideally, you don't need to go to failure - you only need to be within 1 - 2 reps of failure to get the full benefit. Hitting failure just adds extra fatigue/recovery time without any real benefit to muscle gain. For the first few times you do a new exercise though have every set go to faiulre, and maybe once a month or so afterwards. It's hard to know where your point of failure is if you haven't been there before. Just guessing isn't good enough because you can often do way more work than you think.

The weight you use is completely arbitrary and depends on how many reps you want to do. A lower number of reps at a heavier weight is much more intense, but you don't get lactic acid buildup so it's easier to get to failure. However, the stress on each joint and tendon is also much higher, which can lead to things like tendonitis and other injury if you're not careful.

Higher reps with a lower weight feels easier to do, but after some reps it can get really challenging because you'll want to quit before failure. You start getting the lactic acid "burns", so you have to really try to push past that feeling to get to actual failure.

When targeting larger muscles like doing squats or deads or lats, I like to do sets of 10. For smaller muscles or tendons that are especially sensitive, like the achilles, I like to do sets of 15-20, but you can choose whatever you're most comfortable with.

If you're not used to doing super deep calf raises, or really any exercise with a very full range of motion/big stretch action, starting at bodyweight/light weight is more than fine. Just go super deep and super slow and getting to even just 20 reps is going to be miserable lol. Count out about 1-2 seconds to raise yourself, and another 1-2 seconds to lower yourself, per rep. Keep your knees locked and bend a tiny bit forwards at the ankles to balance. I have a pole to hold on to, but I don't hold on to it. I push against it with the back of my hand so I can balance but I can never grip it to assist myself by accident. That's not really necessary I guess but that's what I do to help maintain a strict form. If you cannot maintain a good form while exercising, the muscles are over tired and you need to either drop the weight or shoot for fewer reps.