What's a good starter AR-15?
What's a good starter pistol? SigP320?
Looking to get the fundamentals of it
@David_TheMan gave you some great advice, but I would ask the question what will you be using the gun for? Is it a home defense gun? Are you shooting for personal defense? Are you shooing for fun? Do you just want a cool looking gun.
Once you can seriously answer these questions your next question should be whats your budget? From there will you be able to invest the time to get good with the weapon. There a literally a shyt load of weapons on the market from full size to sub compact. Until you can give us some answers we may not be able to help.
Same thing goes for AR's. There is a low range up to an over the top range for competition. I am a big fan of the Ruger 556. The price is right you can have it for less that $600, but the sky is the limit for AR's. I am a big fan of noveske and daniel defense rifles but they are way out of my price range. AR's are so plentiful there is a something for everyone.
There is a lot of information in this thread. If you can give us more information I am sure people will be able to give you some good recommendations.