Hi there
Firstly, apologies if this is in the wrong section, I just can't see a new player forum.
I'm about to DM the 5e starter kit to 4 people who have never played but have a good grasp of the setting, games in general. I myself haven't played AD&D since the early 80s! And that was always as a player.
I've read and re-read the instruction booklet in the box but I'm still a little confused as to how I come up at the DC value. I understand rolling against AC, and stats, that's all fine. But if a situation arises where a player wants to make a throw against doing *something* that simply isn't covered... do I just use best judgement and make it up 2-19? If that's the case, fine, but the book simply doesn't state it obviously enough for me 
Also, is there anything I can download / print out etc. that would be a good helper for me? cheat sheets, whatever.
Many thanks
It's explained on pp.2-3 of the LMoP adventure book in the section on "Improvising Ability Checks". Basically, you make a judgement about how easy or difficult you think the task would be, then convert it to a DC number using this scale:
DC5 Very Easy DC10 Easy DC15 Moderate DC20 Hard DC25 Very Hard DC30 Nearly Impossible
This is taken from p.58 of the Basic Players Rules (the free PDF download).
It goes in steps of 5. There's no need to go any finer-grained than that.
In general, be very careful about going above DC 15, and keep those under control as well. A PC without any bonuses will only succeed 30% of the time at DC 15. Even a PC with a high score in the relevant stat (say, 16, for a +3) and proficiency (+2 at 1st level) will only succeed 55% of the time at DC 15, which more or less is a coin toss. DC 10 is good for a lot of things because it leaves the chances pretty open for most people with a -1/0/+1 in the relevant stat and no proficiency (for essentially a 50/50 chance), while skilled and dedicated PCs (with both proficiency and/or a high stat for a +4/+5 total) will succeed most of the time (70-75% success rate). Save the higher DCs for truly difficult things that even highly skilled PCs would need to reconsider. Low DCs (i.e. DC 5) can be for things that are trivial for skilled PCs, and offer a low, if not trivial, risk of failure for the rank and file.
One thing to be clear on is that you, the DM, must decide if the action the player desribed for his or her character is automatically successful, automatically fails, or has an uncertain outcome and is thus deserving of an ability check. In the latter case, the DC is what you honestly feel is appropriate for the goal and approach the player has chosen relative to the circumstances of the scene. Think to yourself in the fictional sense: Is this going to be easy, moderate, or hard? Then apply the appropriate number. If you never use anything other than 10, 15, and 20, your game will be fine.
A player cannot choose to make a roll - that is solely in the province of the DM. If a player asks to make an ability check, tell them that you'll decide if they need to make one and all they need do is be clear as to their goal and approach.
Thanks for the advice, all, very helpful!
I've also found through play and analyzing the WoTC adventures that about 80% of the DCs are 10, about 15% are 15 and only about 5% are 20. (For levels 1-6 at least).
Takeaway from this, more success makes players happy and keeps the story flowing.
More often then not I stick to DC's of 10, 15, and 20. Sometimes I just think of how difficult something should be on a scale of 0 - 10 (0 being fairly easy, 10 being pretty difficult) and then add 10 giving me a final result of DC 10 - 20.
thanks aagin, I've d/l the rules now as well (nice of them to make them available!). Was also wodnering, are there are *great* sites out there that are must-reads?