1) Advantage/Disadvantage:
The big thing about advantage is that it allows you to sneak attack. It is not the only way to sneak attack, however. While flanking no longer exists, the rogue has a special rule that they can sneak attack (with melee or ranged attacks!) if, essentially, at least one ally threatens their target.
So if you have friends that will often be in melee, you will usually have targets to sneak attack.
If not, you will need to find ways to gain advantage on your own. The most common way will be via the hide action. This will tend to work best with ranged attacks, since you will usually need to getout of sight of the enemy. Some races can impact this - halflings can hide behind larger creatures (like, say, other party members), while wood elves can hide in certain natural forms of light cover (mist, foliage, etc).
Hiding takes an action, which means at level one, if using it heavily, you will typically end up hiding one round and then attacking the next. However! At level 2, Rogues get something awesome called 'Cunning Action'. This is a bonus action they can take every turn, usable only to Dash (double move), Disengage (avoid Attacks of Opportunity) or Hide. So this makes it much more viable to dart in and out of hiding every round while attacking your enemies.
Other ways of gaining advantage include allies helping you in combat (the new form of Aid Another), or spells and abilities of various sorts. As an Arcane Trickster, you can pick up some of those spells yourself. As an Assassin, you get advantage vs creatures that haven't yet acted, so that can help in round one of combat. Or, if you multiclass into the right class (Barbarian, Fighter), there are various ways to get advantage on a regular basis.
Disadvantage is bad for you because it can negate your own advantage and prevent you from using sneak attack. Concealed enemies cause disadvantage on your attacks. If you are Blinded, Poisoned, Prone or Restrained, you have disadvantage. So those are things to watch out for. Ranged attacks no longer provoke if threatened, but instead have disadvantage up close, so that is also worth keeping in mind with some builds.
2) Optimal weapons:
Assuming you are a standard dex based rogue, the Rapier is always a solid choice. A short sword can work as an off-hand weapon - you don't need any special feats to attack with two weapons. Your off-hand attack won't do a ton of damage, and it won't let you sneak attack twice a round, but it will give you two chances to hit, which can be nice. It does use up your bonus action to take that off-hand attack, which means it conflicts with that Cunning Action ability I mentioned early.
So if you are a rogue that wants to bounce in and out of combat and the shadows, dual-wielding will probably be less useful. But if you plan to roll in and just keep stabbing the enemies your fighter is keeping busy, it can work quite well.
Daggers are lower damage but give you the flexibility to throw them.
3) Assassin:
The big thing for the Assassin is that it is very scary in that first round of combat. You gain advantage automatically on any creature that hasn't yet acted. And if you can actually catch enemies by surprise, you can auto-crit them. (Which doubles the damage dice you roll. Which, with Sneak Attack, is quite scary!)
How useful that is will often depend on the party. Is the party as a whole sneaky enough to occasionally catch enemies by surprise? Or will they be willing to let you scout ahead to try and take some sniper shots at unexpecting foes? If so, the Assassin can definitely pay off. But if the group just charges into each fight without caution, it won't be as useful.
Trickster gets you quite a bit of utility, both in and out of combat - summon your own concealment with Fog Cloud, go invisible, create illusions, charm enemies, etc. Or even lets you pick up a few direct damage spells to help out in non-standard ways for a rogue. But I think it doesn't get too much raw power until later levels. Early on, it does get quite a few cool tricks, like picking locks from across the room, so that can be neat.
4) Wood Elves are a pretty good rogue choice. They boost your speed by 5'. They also get good stats for a rogue (+Dex), can hide easier than most, and gain proficiency with the longbow (a great ranged weapon for a rogue sniper).
However, keep in mind that starting at level 2, Cunning Action lets a rogue double move and still attack (or do other stuff). So even a dwarf rogue can end up moving pretty fast at that point, compared to most other classes.
5) There doesn't seem much involving either of those in the basic rules. Assassins gain proficiency in making poison, but the actual details on that seem to be elsewhere (presumably the DMG when it comes out).
