OK. To start off, CSing is the single most important aspect of the game. So your question is extremely good and a very good one to ask to help you get better. As for a benchmark, Perfect CS is 100 at 10 minutes, but that's very very hard to do unless you are freefarming without getting harassed by your opponent.
What I do when I pick up a new champ is to go into a practice game and learn their auto-attacks and what abilities can get you last hits in a pinch without breaking your mana pool or whatnot. Usually I jungle, but when I do AP mid or bot I can get around 65-80 creeps by 10 minutes, which is fairly decent. Just get a feel for your damage early with boots and with a Dorans blade/ring and make sure you are comfortable with who you are playing.
As for freezing your lane, what it is is keeping the creeps in one spot just before your turret to ensure that you are safe to farm without overextending and risking a possible gank on yourself. For example, let your creeps fight the others while you sit idle and wait for the last possible second where you can farm, you will keep the creeps in the set spot that you want. As to when to freeze the lane is when you need a gank, and you have to force your lane opponent to push with their creeps so they dont get denied XP or potential gold.
If you get a successful gank or kill on your lane, always push your lane to their turret. Don't hit their turret, just push the creeps to it while they are not there. It denies them gold and XP and forces pressure for them to get back to their lane, even to make their jungle help their lane out and force him to steer away from his farm/other lane ganks.
So, in summary:
- Get a feel for the damage to what creeps you are last hitting, the auto attack animation for set champions
- Freeze when you need a gank
- Push hard when your lane opponent is forced back or is dead is the short version.