I'd also like to mention, when you reach the correct account level and if you have a tier 6 ship, you may want to consider playing operations. They give decent rewards including 10 point captains for a few nations, premium time etc.
As for DDs, there are some brilliant lines that serve different purposes.
Russian/USSR DDs have good guns, fast top speed, poor concealment and their torpedoes are short ranged.
American DDs are sort of "hybrids", they have decent guns with fast reload but the shells are quite floaty, making long range gun duels quite difficult against anything relatively fast. They can't stealth torp* until tier 7. Nice team players because of their long lasting smoke screens. They also have nice AA for a destroyer.
*Stealth torp means you can launch torpedoes outside of your detection range. (For example, detection range of 6km with 8km torpedoes)
British DDs are similar to American DDs as the British DDs are also hybrids. Guns are quite floaty just like the American ones. They can single fire torpedoes which if used properly are amazing.
Some of them have really good concealment, for example the Lightning (tier 8 UK DD) can get something like 5.5km concealment if built for it.
From tier 6 (I think) they get hydro so they can spot other ships and torpedoes whilst in their own smoke. I think the higher tier UK DDs get a heal too, it's a consumable that slowly regenerates health points whilst its active.
German DDs are kinda like hybrids too. They have decent reload speeds on the guns and torpedoes but the HE (High explosive) shells don't do much damage, their torpedoes don't do much damage but have nice reload speed. These also get hydro from tier 6+
IJN/Japanese DDs are split into 2 lines. The one that ends with Shimakaze is the torpedo focused line and the alternative line is focused around guns from tier 8+.
IJN torpedoes have very high damage and are decently fast but they are detected quicker by enemies. (Shimakaze has a 20km torpedo option but they are spotted from 2.5km away, allowing for a lot of time for the enemy to react and dodge.
French DDs are similar to Russian ones. Very gun focused, extremely fast (some of them reach over 50 knots) The French DDs don't have a smoke screen.
Hope this helps!