okay so I met this guy who saw me on the beach... and he took my number from I don't know where ..he keeps saying that he wants me to work as a model ... I am 179 cm (5'9")and 75 kg..and I totally think I'm fat! so I don't really believe the guy ...the problem is that I would love to work as a model , just that I'm not sure ... am I fat!?
Modeling isn't a homogeneous market, so for some markets, yes, you're fat (50 kg is fat for haute couture, especially in Europe), but for other markets you'll be okay(lingerie, swimwear, retail advertisement, athletic).
Your agent's job will be to know where he can sell you, so if he does his job, and if you're professional with good habits, you should have nothing to worry about.
First thing is to check him out. Ask for his full name, company he works for, and some sort of MAILING address. A lot of jerks try to pick up women like this and they prey on the "not 100% perfect" women because (sorry to say) they are the easy targets.
If he doesn't give you all that information he's lying.
If you're getting modeling offers that should be proof that you don't need to be self conscious =)
You don't sound like you're fat based on height and weight but those are just numbers, what really matters is how you carry your weight. I don't know about runway modeling but glamour modeling could be for you. You don't necessarily have to be a skinny couture model. Just make sure he's legit and not some weird predator.