Always do small dungeons when you're alone, that is the quickest and most efficient xp.
As you level up more floors become available to play on, but you will eventually hit the point where you cannot go to the next floor but you have already completed all of the floors once. When that happens, you have to reset your current progress (prestige) which is an option on the menu right before you enter a dungeon.
If you plan to actually play legit, I suggest you also upgrade your ring (which can be done by right-clicking your dung ring while you're inside of a dungeon).
Good luck! I find dungeoneering quite fun, hopefully you do too or else 1-50 will suck!
PS: if you smoke the danks, I highly recommend doing so before every few dungeons
