While it's fine to mix and match abilities as you wish D2R Items, it's generally best to make a plan and then invest accordingly. You're only given a certain amount of skill points available and therefore you will not be capable of acquiring everything on the skill tree. And even if you do spend money on a particular area of expertise.
That may dictate what other abilities you can choose to use due to skill synergies, which are listed when hovering over a skill. For instance, the fundamental Raise Skeleton skill for Necromancers has a benefit from Skeleton Mastery and Summon Resist. This makes those important skills for summoners to pick.
Resurrected includes a respec feature which means you're not completely locked into the choices you make, but there are some limitations (you'll receive respec options for free while playing but the subsequent ones will require you to gather certain resources). This means that you shouldn't anticipate being able freely experiment to the level as you could in Diablo.
One of the gameplay mechanics that could be a bit cheap d2r items confusing is stamina. You can sprint for a short amount of time before exhausting of energy and being forced to walk. You can toggle between walking and running on your own to preserve the endurance of your body and to ensure that you're not completely exhausted when you suddenly have to put some distance between yourself and your enemy.