I'm on WoWs now guys, and i'm a noob.

General guidelines for ammo choice:

When driving a DD, almost exclusively use HE. Your AP isn't gonna do much to BBs, or cruisers at range. Against other DDs, your AP rounds are likely to either bounce or overpen. Now, if you happen to get a broadside cruiser within 8 km, use the AP (some exceptions apply. Cleveland especially is a tough nut to crack for low caliber guns.)

On the opposite end of the spectrum, when driving a BB that isn't a tech tree British ship, you almost exclusively use AP. Even against DDs, AP isn't a terrible choice, especially if it's what you've already loaded. Even an overpen will take 5k+ off a DD's health, and your reload is far to long to make changing ammo worth it until after firing. In a non-premium British BB, spam HE. If the target is going broadside, it's okay to queue up some AP rounds, especially if it's a squishy cruiser.

Cruisers are more difficult. You'll be fine spamming HE in most situations, especially at range or against BBs. DDs should be shot at with HE. Enemy cruisers are where it gets tricky. Broadside cruisers at close range (<10 km) should always take AP. Some cruisers have armor so weak that even angled will take AP, such as Emerald and Omaha. Some cruisers have better guns, and can shoot angled cruisers with some success (the 203 USN cruisers with Super Heavy Shells from the New Orleans up are like this). This is where knowing what your guns can do, and the armor of the enemy becomes key.