As people above me have stated, electric powered guns are the way to go for new players. If you plan to use a pistol, most often people use CO2 for them. With an electric gun, if you plan to play for more than 4-5 hours at a time, you may need multiple batteries.
Make sure you know all of the rules at the field of your choice along with secondary options for arenas/fields. Different fields have different rules and it would be best to get adaptable gear so you can make smooth transitions between the fields and your play-style if you choose to do so in the future.
I made the mistake of buying an incredibly cheap electric AK74U with horrible parts and garbage stats for my first gun- it was $60. It lasted two games (8 hour games, so 16 hours total) and then busted. My second gun was an M16 and ended up having really nice power, high fps. The field I normally play at allows it as a rifle, so I can shoot people at whatever range I want; even barrel-to-skin. However, the three other fields in my area classify it as a DMR, so I must be 50 feet away from the enemy to shoot them. It was disheartening to hear that I'd put myself in this box after a $250 purchase.
For starters, all you'd need is your gun, a face mask/goggles (whichever your field requires), and a place to hold your mags. I carried my mags in my pants pockets for four months Some fields also require muzzle covers but they would probably provide them. As well, most fields require you to sign a waiver- mine does it online.
I am not sure if this is common, but all the fields in my area also base teams off of your loadout color; generally green vs. tan, so you and your friends may also need to consider than when planning.
I buy my weapons second-hand on BST forums and from word of mouth. However, if I were to buy a specific brand, I would choose Evike and Krytac for rifles and Tokyo Mauri for sidearms. Unfortunately, it's an expensive hobby.
As for indoors vs outdoors, it seems that indoor play attracts more "high efficiency" players. I would compare it to laser tag or paintball. It has held much more action (in my experience) than outdoors gameplay and hosts a lot more younger players. Outdoors, on the other hand, especially fields that are 20+ acres, is home to military simulation style play. In my home state, entire town centers are built solely to host massive simulation events. I started off airsoft playing indoors with the speedsoft crew and absolutely hated it. Once I transitioned to the field I currently play at (over 100 acres), I found that i enjoyed it much more.
After such a long-winded and off-topic post, I suppose what I mean to say is that airsoft offers a lot to love for everyone, you just have to find it.