Theres not a whole lot that will work in a 30 gallon except some of the smaller species of land turtles and tortoises.
In a 30 maybe you could keep a box turtle - (eastern,desert,texas will work but not florida of gulf coast,they get to big and not asians because they need water for swimming) or thorn turtles
For tortoises - in a 30 your pretty much limited to russian's, or one of the hingebacks (bells,holmes etc..) or stars (stars are expensive)
If you want to upgrade later on most tortoises grow pretty slow - greeks,iberians,forsteins,elongateds,marginateds,leopards,hermans radiateds and a few others all grow pretty slow and don't get to huge.
If you want to upgrade later the list of turtles you could start out in a 30 is pretty long.
The two main things to consider in useing a 30 gallon tank:
1) if you go aquatic turtle and have half land and half water is either big enough.
2)if you find a smaller species make sure the tank is wide enough for both to walk side by side - if not and on flips over and can't flip back you'll lose one
Hopefully that helps you out some.
If you find a specific turtle or tortoise and need some info maybe I can help you out a little better.Kind of hard to give general info when theres thousands of different species.