Hi... I am new to D.L and I want some advice, need a good deck and some advice on how to level up characters and get gems.
If you started with Seto Kaiba, I'd advice you buy Chazz Princeton's Dragonic Fusion And Kaiba's Dragonic Force Structure Decks. Use P. Of Felgrand and Dragon Of Creation to special summon your big guns. Use Arkbrave's effect to summon from ur graveyard also set dragon's rebirth immediately when it comes to your, keep your deck at beween 20 and 24. enemy controller and sphere kuriboh are good protection cards for beginners
If you want a cheap good deck I'd probably go with alien (Abyss Encounter box).
As for leveling, the only way to gain experience is dueling, so duel a lot (and win for more xp), pay attention to double xp events.
And about gems, you get them mainly from events, leveling up chars, and completing missions.