what do you base your roll on? Do use stealth, slight of hand, straight dex?
For rogues, it says you can double your proficiency for thieves tools. But how do you calculate proficiency in it?
I have a bard/sorcerer with urchin background, which gives thieves tools. But I don't know how to use it
No it's a TOOL and has its own profiency. If you are proficient in them when you pick a lock (for example) you make a tool check (normally with Dex) and add you Proficiency bonus to it.
Your Proficiency in anything is your + like at first level it's +2. That applies to weapons, tools, or skills.
God forbid the gm decides you need thieves tools with a cha check. Dont want to know what kind of lock needs that.
Zarton wrote:
Mad_Jack wrote:
If it's a lock that's rusted solid, a Strength check might to appropriate. Fail badly and your tools break
Or, if it's a really easy lock but you're trying to impress the beautiful princess with your dashing good looks and dazzling skill, you might make a Charisma check. Fail badly and the lock opens anyway but she thinks you're a jerk.
Wisdom check if it's not even locked but you poke about inside it with your screwdrivers and discover the poison needle.
Proficiency Bonus applies to all those. 
Mad_Jack wrote:
Zarton wrote:
CFK wrote:
"You don't need a tool for that... or do you?"
Perhaps not. But without one, there's not a lot of point to picking that particular lock in the first place!