Rainbow Six Siege is a game that rewards being in the right place at the right time just about as much as it does FPS shooting skill. As a Defender, you have the task of keeping the thing that the Attackers want away from them, whether that be in the Bomb, Secure Area, or Hostage game mode. There are a ton of different ways to accomplish this, but in general, there are some things your team should always be doing in order to succeed on Defense:
10 ROAMERS: KEEP ON THE MOVE
As a Roamer, it's usually better to stay on the move rather than chill in one spot, especially early on in the round. Keeping on the move makes it immensely more difficult for Attackers to box you in, and for the same reason, makes it easier to catch an Attacker off-guard and get an easy kill. There are definitely situations as a Roamer where it's more useful to pick a spot and either wait for an Attacker to show themselves or just hold a sight-line, but as a general rule, a Roamer should always be utilizing rotation routes throughout the map.
9 ANCHORS: HOLD THE CRUCIAL OBJECTIVE SIGHT-LINES
An Anchor's job is the exact opposite. Anchors are there to keep an eye on the objective site(s), and to hold the crucial sight-lines that keep the Attackers from having an easy entry. Anchors should be that wall that the Attackers come up against that hold them off long enough for Roamers to have the opportunity to come and flank the Attackers.
8 PICK CERTAIN OPERATORS FOR CERTAIN OBJECTIVES
It's very important to think about the objective site that you're being given before you make a decision on the Operator that you're going to play. Sure, certain Operators like Jager or Lesion are useful in just about every single objective site in the game, but the same can't be said for every operator. Mira, for example, isn't going to be useful in objective sites with easy Attacker sightlines from outside the room. In certain objective sites she's a boon, and in other objective sites, she's a detriment. The same is true for a lot of other Operators.
7 HAVE A PLAN WITH REINFORCEMENTS
Just like you should have a plan with picking certain Operators, it's important to have a plan on how to reinforce certain objective sites. Some objective sites need to be sealed off with reinforcements as much as possible, but others should have certain walls/ceilings left open to be easier defended (especially if a Mira is involved.) For example, on Border, if you're defending the Server Room/Workshop, it may be better to open the ceiling hatch above the Server Room in order to combat the hallway entrance.
6 ATTACKERS ACOG ADVANTAGE
Some Defenders have access to Acog sights (and more Defenders may be getting them in the near future), but every single Attacker except for Nokk has access to an Acog. This means 99% of the time, if you're in a fight with an Attacker and you don't have an Acog, they do, giving them a distinct advantage in any fight that isn't close-range. It's important to keep this in mind; it doesn't mean to never engage, but understand that the enemy should have an easier time lining up a shot than you will.
5 PLAY THE WHOLE MAP
Something new players tend to do a lot is stay as close as possible to the objective site(s), without much of a plan and regardless of the Operator that they choose. While the objective is obviously critical, staying as close as possible to the site isn't always the best option when it comes to defending it, especially if there are already one or two Defenders in the site already. Spread-out, combat Attackers from spots they aren't going to immediately expect, and you'll notice a lot more success down the road.
4 CONCISE COMMUNICATION
Communication in Siege is vital, but over-communication can be as crippling to a team (or more so) than not communicating at all. Giving information out to your teammates should be quick and to the point; don't give out a stream of info that takes longer than a few seconds to say. Hearing footsteps and other critical sounds is paramount in Siege, and sometimes useless information can drown out a crucial sound that would have made the difference between a victory and a defeat.
3 SPREAD OUT/DIVERSIFY SIGHTLINES
There's rarely a reason for two Defenders to hold the same sight-line. Even if you're in the same room as a friendly, make sure you're each watching a different entry point, provided you're in a room with more than one. (Which, just about every room in the game has multiple entrances/sight-lines.) If multiple people do end up having to watch the same area, try and do so from different points. Grouping up is a good way to give an enemy an easy multi-kill.
2 SWITCH UP TACTICS
Try not to employ the exact same tactic twice, even if you end up on the same objective site and even if it worked beautifully last time. Most teams are going to recognize the same tactic and adjust their own this time, usually countering effectively. Have multiple strategies up your sleeve for each objective site in the game, and rotate through them as best you can throughout the match.
1 KNOW YOUR ROTATION LANES/HOLES
Lastly, always have in mind the quickest/easiest way to rotate around the map, even if you're an anchor. This really just comes down to map knowledge which can only be gained by play-time, but the difference it makes is huge. It'll help you to maneuver all around the map in multiple ways if need be, it'll help identify areas Attackers are likely to try and move to if they've been rebuffed from a different area, and it's going to help net you tons of flanking kills on Attacker that you've outmaneuvered without them even knowing that you could.