[JD¹³]

You have to start somewhere. I'd say find the weight you can comfortably do 10 reps of and concentrate hard on your form. Flex and squeeze your chest through the entire rep and hold at the top for a full "one-onethousand". Arms down square to 90 degrees so you're feeling the stretch at the bottom end of the rep. If you do weight that's comfortable for you and focus on your form I guarantee you will see increased strength in as little as 3 weeks. Once you've got the basics down you can start to push and see what you can really press without the risk of hurting yourself.

EDIT - And that goes for any exercise on any muscle. Do your research, watch YouTube videos of proper technique before you try something new. I can't count how many guys I see rocking their bodies on the seated row machine, yanking back with their upper body with weight that's way too much for them, and pulling the cable barely half way to their abdomen and thinking they're strong. I'm waiting for someone's lower back to explode. By doing exercises totally wrong you're only cheating yourself while looking like an idiot in the process.