Dude, don't stress about being the skinniest, weakest in the Gym. That was me last year, when I weighed 55kg (~115lb) and was 6'. I know exactly how you feel, and how embarrassing it is. My worst moment was my first bench press, I needed a spot to the bar with no added weights. That's embarrasing.
Keep going though and you'll soon get over it. For one you'll be lifting bigger weights, and two you'll see that no in there really gives a shit how little you can lift. Plus, if you push yourself hard enough, you'll be so wrecked physically that you won't give a damn what others think of you.
Some advice though is to put in as much effort as you can, keep going till you can't lift that next rep. Stay away from machines as much as you can, for example, you mentioned the peck deck? Look up dumbell flyes, I find them easier, and they hit my chest much more effectively.
And finally, most importantly; do NOT be embarrassed by how little you lift. Do NOT sacrifice form and technique to lift a heavier weight. Do the best you can, and you will see results, and so will others.
The rest is up to you. The fact you're posting here, and have signed up to a gym proves that you're determined. Good luck.