It would be helpful to know where you're at in the game and what you're struggling with. Do you need help with the Rogue class basics, or are you trying to find a powerful build path?
If you're a complete newbie, playing a Rogue is going to be pretty challenging because you have to understand a bunch of different mechanics: Sneak Attack, stealth, dual wielding, finesse/light weapons, how to leverage bonus actions, how to maximize AC, etc. If the in-game tooltips aren't explaining things well enough you may want to consult a guide; I can try to find one for you if you'd like.
As far as battle tactics, your basic goals as a Rogue are to get your Sneak Attack off every turn possible and avoid getting hit because your HP is low. I find that it's a lot easier to play Rogue as a ranged weapon user instead of melee; you can stand back and let your melee tank (i.e. Karlach) run into the fray, and then get your ranged Sneak Attack in against any enemy in the tank's melee range without having to worry about hiding. Helpfully, Astarion is a high elf, which means you have proficiency with Longbows in addition to the Rogue's normal ranged options.
At level 3, the Thief subclass is the most straightforwardly powerful because you get a second bonus action each turn, which can be used a lot of different ways to great effect.
At level 4, your first feat should probably be Moderately Armoured, which gives proficiency with Medium Armour and Shields plus a point of Dexterity to get you to 18. This is especially great for a ranged-focus Rogue because, unlike "real" D&D, you can get the benefit of wearing a shield while using a ranged weapon, so it's just +2 AC with no drawback.
Hopefully some of this is helpful. If you're farther along than I think you are and want tips on maximizing damage output in the mid-late game I can provide those as well.
