I'm going to give you a short step by step demonstration of what I know!
One of the most important assets of this game is sound, you need to be hearing what's going on more often than seeing what's going on. This game has one of the best sound mechanics ever, and that's why you should use it to your advantage! Be sure to use headphones or earphone to hear all around you.
This game is all about knowing, it's not about your aim, it's about how you play, and to play like a professional, you're going to have to rely on information.
Knowing where your enemies and objectives are, is the best thing you can have in siege, rushing in is the worst tactic, if you're going to push an objective, always ask your teammates if they know anything about the positions of your enemies. And this is where drones and cameras are important.
Use them to your advantage, this doesn't always mean to mark your enemies though, as this can reveal your hidden drones or cameras. Marking your enemies is still extremely usefull, don't get me wrong.
Operators are the characters of siege, if you will. Each with their unique ability, which are (almost, looking at you Tachanka) all as equally usefull.
Be sure to read about your operators, know what you're dealing with, and know what your job is!
Destruction and penetration:
This is something way too many players forget, this isn't Call of Duty... You don't need to show yourself to get someone, or use a door to get somewhere. You have explosives, shotguns and all kinds of breaching utilities, feel free to make your own entrance! This is also where intel comes into play, if you know where someone is, you can always shoot at them trough a soft surface near them, marking someone helps extremely with this, as you can see exactly where they were at the moment they were marked!
This is also something an equal amount of players forget, the game isn't about killing, it's about completing the objective, if you have the objective room in your control as an attacker, this can guarantee you a win. Just plant the defuser, extract the hostage or secure the biohazard container! This is also why defending the objective is a must for defenders, if an attacker planted the defuser and is watching it from outside, this is almost a guaranteed win!
Time is also important, running out of time will always result in a loss for attackers, but planting the defuser or staying in the biohazard container's room will result in time freezing, altough this does not count for the hostage.
Now enjoy the game, and happy breaching!