If you just started taking working out seriously around 3 weeks ago, don't even worry about the weight. Just set it up to what you can do, increase it slightly, and just work with what you can lift.
Over time you'll definitely notice you can lift more and more weight. When I first started going to the gym, I couldn't lift half the stuff that I lift now. It just takes time.
And if you don't go with a friend..I'd recommend finding a friend to go with. When I work out with a friend, I push myself harder and find it way more fun. It's also good to know you have someone there to encourage you to get those last couple reps up, or someone to spot you if you needed.