Rainbow Six Siege is a very complex game. It takes a little while to feel like you have your feet under you, and the game is arguably at it's hardest to learn while Attacking. New players seem to struggle with Attacking more than they do defending, and that's probably because Defenders have the opportunity to feel comfortable in the map and ready themselves before the Attackers get a chance to move in.
Maps in Siege are complicated, dynamic environments, and Defenders that have a lot of experience are going to use that to their advantage to catch Attackers off-guard. Here are some tips to get you a bit more comfortable Attacking in Siege:
10 ROAM CLEARING
As a newer player, Roamers are generally the type of Defender that you're going to be dying to quite a lot. Roamers are the Defenders that are constantly moving throughout the map, getting flanking opportunities on Attackers and generally sowing discord through your team.
Luckily, there are Attacking Operators that are designed to combat Roamers effectively. Operators like Jackal and Nokk, for example, have gadgets designed to give them equal footing against Roamers. Jackal can track footsteps, giving away the location of Roamers no matter where they are on the map, and Nokk can move quickly, silently, and invisibly in regards to cameras, allowing her to sneak up on Roamers and give them a dose of their own medicine.
9 CUT OFF ROAMER'S ROTATION LANES
Another tactic that will help immensely when dealing with Roamers is cutting off their rotation lanes. There are certain pathways through maps in Siege that are veritable highways that allow Roamers to move across the map pretty easily and safely.
Once you've learned the maps pretty well, you can bring Operators that have claymores to cut off a doorway or window from a Roamer, or bring an Operator like Nomad or Gridlock, who's gadgets specialize in cutting off pathways/rooms.
8 HARD BREACHING
On most objective sites in Siege, a Hard Breacher is going to be quite the benefit. Hard Breachers are Attacking Operators that have gadgets that can destroy reinforced walls/hatches, like Thermite, Hibana, and to some extent, Maverick. Opening up these walls/hatches can provide sightlines that Defenders really don't want you to have, and can give much easier and safer access to the site.
Keep in mind that Defenders are usually going to have one or any combination of Bandit, Kaid, and/or Mute on their team to make reinforced walls more difficult to destroy, meaning it's beneficial as an Attacker to bring either Thatcher, Maverick, or Twitch to deal with their gadgets.
7 USE VERTICAL PLAY TO PUSH OUT ANCHORS
Many Anchors (Defenders that stay in the objective site and hold crucial sightlines) are going to stay in relatively the same position throughout the round. As an Attacker, there are many objective sites that you can get above or below and still combat these Anchors while staying in relative safety from them.
Many floors/ceilings in Siege are destructible, meaning they can be shot through. Learning exactly what is above/below you in every map in Siege allows you to simply fire straight up/down from a location and possibly get an easy kill on an Anchor that thought they were safe.
Good drone-play is a very crucial part of Attacking successfully in Siege. Drones allow you to get a view of a room without being in all that much danger and can give you the info that you need to take control of that room relatively easily.
Moving in a group of two or three throughout the map in Siege means one person can use their drone to scout out locations they're about to move to, while the other(s) cover them. Information is critical in Siege, and drones are an invaluable tool in acquiring it.
5 SPREAD OUT/ATTACK FROM MULTIPLE ANGLES
Rarely do you want the entire Attacking team to be group up together. Getting multiple sightlines and moving in from different angles is going to make a world of difference, and can force the enemy to make a choice as to who they want to engage, giving the group that the Defender(s) aren't currently looking at an easy kill.
Move in from different locations simultaneously, and it's likely the room you're moving into is going to be enemy-free relatively quickly.
4 ALWAYS BE ON THE LOOKOUT FOR TRAPS
There are a lot of Defenders that can set traps in Siege, such as Lesion, Kapkan, and Frost, just to name a few. These traps make a world of difference, and running into them is going to definitely lower your chances of success. Some of them can be difficult to see; Lesion's Gu Mines, for example, are cloaked, making them tough to find unless you're actively looking for them.
Ela's Grzmot Mines can be placed in such a way that they actually can't be seen until you've moved through the doorway they're trapping. Droning ahead is a nice way to spot traps like these, and getting rid of or avoiding the traps can go a long way in ensuring victory.
3 PUSH THE PACE
As a Defender, time is your friend. The same cannot be said for Attackers. Defenders, especially Roamers, are going to try and waste as much of your time as possible, because the more of your time they waste, the less time you have to take the objective site.
Keep the progress going forward as an Attacker, and don't spend too much time on one problem. Once it gets down to the last few seconds of a match, the round almost always ends in the Defenders favor simply because all they have to do is run out the clock.
2 KEEP COORDINATED
Coordination is key in every phase of Siege. Make sure everybody is on the same page in regards to where they're moving in at, and what angle of the objective they plan on Attacking. Call out Roamer locations, what Operator they are, and where they're moving to.
Call out any traps you find, even if you've destroyed them; this helps to let your teammates know how many traps could be left. A well-coordinated team in Siege in nigh-on unbeatable, as long as everybody is on the same page.
1 CONCISE COMMUNICATION
That being said, over-communication can be a serious problem. Drowning out all of the game sounds with communication that isn't necessary is going to lead to a lot of easily avoidable deaths, as people won't be able to hear the relevant communication, and won't be able to hear of any encroaching footsteps. Keep your communication concise and to the point.