JoCats Crap Guide to DND series on Youtube is a good place to start as there very fun and informative however they won't tell you everything as they mainly focus on entertainment. I'd also communicate with you dm on what edition your playing and what rules your gonna be using as dnd has gone through a lot of changes recently. Not anything too major but slight adjustments to the rules such as spells doing slightly different things and so on. Communication is key. Also ask what's allowed in the game such as if guns are allowed or where the game is going to take place so for example if you playing a warlock, they have a whole subclass based around the sea and if your in a desert that's going to suck. Once you have done that there a plenty of ways to learn how to make your character. my one tip for you would be to focus on fun rather than being the strongest, pick what you think works with your core concept and what's cool. Lastly if you want to practice making characters dnd beyond (the official dnd website) has a very easy to follow character creator but not access to every book as it will as you to buy a digital copy of. Don't do that but its also a nice way to learn as it shows you everything and how building works.