Here's a warm up calculator.
Starting Strength Warm-Up Calculator
You can use that to determine your warm up sets/work sets.
As for deciding what weight you should be using per lift, it should be the max weight that can. If you stuggle a bit or fail on the last set, that's ok. Just keep at that weight till you can do it with ease. If you're not even coming close to finishing, then it's a better idea to drop a bit of weight off the bar. It's all just a matter of time before you'll get it fiqured out.
If you're just starting out, use lighter weights untill you have proper form.