Pinpointing your starting weight can be intimidating if you're not a gym junkie. To get started, pick the highest possible weight you can lift with good form for 10 to 12 repetitions. You should just barely be able to lift the 12th one in a controlled manner. That's the goal: Studies show that 10 to 12 reps is the zone where you build muscular strength.
To find your 12-rep weight, start with an empty bar to familiarize your body with the range of motion and coordination involved. After that, load the weight disks conservatively, trying out lifts with the ascending loads, until you find your 12-rep weight. Load the least weight for those lifts that challenge your lower back (dead lifts), which is especially vulnerable to injury and requires extra attention to form, and always work with a spotter at unfamiliar motions. When it comes time to increase the weight, wait until you can do three sets at 12 reps without crying for mercy.