You only need a concentration check if you're distracted, say by a storm, or weather. And a touch attack spell requires a successful hit against an opponent's touch AC to resolve, otherwise it's wasted.
From the PRD:
"If something interrupts your concentration while you're casting, you must make a concentration check or lose the spell."
-http://paizo.com/pathfinderRPG/prd/magic.html#_concentration
Otherwise, I agree with Lathiira and Kazaan.
To clear up all the related questions, here are some basic, general rules all spellcasters should know. Learn these, and learn the details of the specific spells you're preparing, or at least get familiar and learn where to look it up quickly, and you have the majority of it covered.
1. Normally, casting a spell doesn't require any roll at all. For example, if you cast Invisbility on yourself, you simply go invisible. Done.
1a. There is a chance bad things might happen during the time you are casting, such as you might take damage, or you might have significant distractions like being on a moving horse or ship, or a number of other problems that are detailed in the Concentration section. To avoid losing the spell with no effect, you need to make the appropriate Concentration check.
1b. If you cast a spell within reach of an enemy, that enemy can attack you unless it's unarmed. This is an Attack of Opportunity and it always applies to spellcasting and a number of other things that provoke Attacks of Opportunity - these are explained in the Combat section. This basically lets them take a free shot at you and if they hit you, the damage forces a Concentration check as I mentioned above. You can try to avoid this risk by Casting Defensively which is explained in the Combat section and requires a Concentration check.
2. Unwilling targets can usually try to avoid the effects, but you spell still automatically hits willing targets unless it specifies otherwise.
2a. Some spells require you to touch an enemy. Unlike stabbing them with a sword, touching them only requires minimal contact and does not need to penetrate their armor, so you don't need to hit their AC, just their Touch AC.
2b. Some spells use the same Touch mechanic but you can do it at range, like firing a laser beam (a "ray", for example). It works just like a touch spell but you can stand farther away from the enemy, and other factors might apply as with all Ranged attacks (firing into melee, cover, etc.), all of which are detailed in the Combat section.
2c. Some spells don't require you to target them at all. You simply decide where or what they affect, and they do. However, unwilling targets often get a chance to resist some or all of the effect by making a Saving Throw. Your spell description says what kind of Saving Throw, if applicable, and what the reduced affects are, if any.
2d. There are other possibilities, but they are much less common and usually fully described in each spell that uses them.