Ultimate tip:
Arkham Horror Rules Summary
The official manual, while comprehensive, is shit. Very hard to follow. Take the above PDF and print out the first two pages (base game rules summary) double sided onto card and then laminate it. That's your new rulebook.
Soloing is a great way to learn the rules, which in turn makes teaching others easier, but it makes the game near impossible to win if you're new to it. I recommend making it easier by allowing only doing the Mythos phase every
other round, as that gives you a fighting chance.
It's a complicated game and so when introducing it to friends you have to have your shit down, else they'll just be put off. If you get a group of you working together there are few boardgames as rewarding and fun, but you as party host need to put in that rule familiarisation legwork. The summary pdf helps immensely with that.
I personally recommend going all atmospheric and doing corny American film noir accents as well.
And just make sure everyone knows the average game is four hours long.
If your crew isn't used to long, co-op boardgames, a fantastic introduction would be
Pandemic which is much less rules heavy and focuses on a theme everyone understands.