Take advantage of his neutral game and learn his range well. His 5B, 5C, and 2D have amazing range. That alone will help make your battles that much easier.
Also learn on how to safely end your pressure strings with him. Mix-ups and gimmicks and all of those fancy things can be learned in time as you get better and play the game more. The post above yours I touched on pressure, that stuff all applies too.
Midscreen, try your best to end in Inferno Divider Axe Kick, in order to keep them right in front of you. Hell's Fang is fine if you're pushing them to the corner though, but usually you should only use it off max range confirms. In the corner, you should almost ALWAYS go for Hell's Fang over 22C, since it gives better oki. As far as damage goes, if you're hitting 2-3k midscreen, and at least 4k midscreen, then you're doing fine, but you should really hit up the combo thread for some of the hard hitting fatal and crouching combos (as well as Blood Kain combos).
Be sure you also learn your methods of catching rolls for people who want to escape your oki. 2B and 2C both work REALLY well at catching tech rolls. This, however, really comes down to experimenting with your best options in training mode, and then conditioning your opponent in battle.
That's really it as far as generally-playing things go, Ragna is a basic, but extremely solid character. Play good neutral, smart offense, and strong defense, and you'll be fine. Learn your combos and make every hit count, no matter how random.
We have a critique thread if you have any matches you'd like us to go over and help you out with. Other than that, any specific things you'd like to know? It's kinda hard to answer your post without any real specific info you'd like to know.