WarOwl gives quite a few tutorials on the basics of how competitive CS:GO works and the various weapons. 3kliksphillip gives more of a technical side of weapon statistics though. adreN is a former competitive ESEA player who was on Curse and iBuyPower and gives a lot of basic tutorials on map positions, AWPing, etc...
Also, as far as aim goes, learn how to tapshoot. Especially when first getting used to the game, try not to fall into the habit of spraying your weapons. Arms Race doesn't help since you won't be touching most of those guns anyways. I would recommend playing online Deathmatch on a map with lots of encounter points like Dust 2 or Nuke. Learn how to use AK47, M4 of your choice, AWP, P250, CZ75, P2K/USP, and Glock. M4, AK, AWP is just the basic meta that the game revolves around, whereas the P250 and CZ75 are great eco weapon choices. The starting pistols actually pack a punch against buying players if you can connect the headshots and knowing how to use them will make getting the momentum started from pistol round or save rounds even easier.
Once you get more used to weapons, you should probably learn some maps. Bots are stupidly easy, so I would say don't use them to practice unless it's basic aim practice and learning the maps. If you want to start somewhere, you'll want to learn the standard competitive maps like Dust 2, Nuke, Inferno, and Mirage. This means positions, map callouts, team strategies, nades, wallbangable areas, boosts, jumps, etc...
As far as modding goes, there's the standard custom servers like Surf, BHop, Course, Deathrun, Jailbreak, etc... I personally don't find any of them too fun except for Surf, Bhop, and occasionally Scoutknivez.
Here's some basic tips that'll last you a lifetime:
Buy head armor
Don't use Bizon
Knife people for 1200 dollar kill reward
Learn spray patterns
Don't move and shoot
Learn to jigglepeak and stutterstep
90% of the time, crouching is a bad idea