What's the best guitar for a beginner?

Can I put steel strings on my classical guitar?
Absolutely not. You're very likely to ruin the guitar and maybe hurt yourself when the neck breaks off! they're a completely different tension on the neck.

How much does hot and cold affect the guitar? and do I need a humidifier?
So unless you live somewhere where it gets REALLY hot or REALLY cold, then you're probably ok if you use your head. Leaving an acoustic guitar in the hot sun for a whole day will probably ruin it, leaving it out in the snow might not be too good either!

If it gets very cold and you use central heating, the humidity can drop and you might like to look into a Dampit or similar for acoustic instruments.

I've never had any problems with hot, cold or humidity and never used a humidifier, but if in doubt, try and get some advice from your local store who should be able to advise you.

Can I use a plectrum on my classical guitar?
You sure can! That's how I learned. They're not really designed for that but nothing should get damaged and it's fine for learning. If it's an expensive instrument you might look into getting a Flamenco Guard to stop the pick damaging the wood, but I think war wounds can make a guitar look cool and so I never used anything like that!

I bought an electric and an amp, what should the settings be?
Turn all the knobs on the guitar all the way up, and adjust your sound on the amp. Get a sound you like. If you want to experiment a bit, try changing some of the guitar controls and see what happens, you shouldn't damage anything! The all up on the guitar and adjusting the amp is the kind of default settings for most players!