How to last hit like a pro?

I'm new myself and I've found that I don't use abilities early on to help farm. Mana is just too precious / regen too low early on. Later when you know you've got the mana (and time) to spare, go ahead and wipe out most of the wave with an ability.

As for learning to last hit, play some bot practice matches with a few heroes to learn their attack animations as others said. The more familiar you become with the timing, the closer you can cut it to minimize your harass damage.

Use this commands.
Find them in E:\Steam\steamapps\common\dota 2 beta\dota\cfg\config.cfg
Open config.cfg with Notepad/Wordpad.
The commands are:
1)dota_player_units_auto_attack 0 You will only attack creeps when right clicking them or issuing the "A" command, so you wont need to use S or H to avoid auto-attacking.
2)dota_force_right_click_attack 1 You will be able to attack your creeps with the right click command if health permits deny. So using this on your creep it will always try to deny him, if the creep has full hp and you click him nothing will happen. With this command on 0 value when your creep can't be denied you will issue a move command with the right click.
Don't insert the commands in the console because they wont save in the config.cfg and you will need to do it every time you exist and enter the game.
Hope it helps.

This is great but I have a problem....

How do you actually do this? I am very new to dota2 and I haven't played dota1. Should I change the config.cfg then save it? Do I need to do this ingame? If this is done ingame how do you do this? What do I actually press. Please be very specific because I am really new to this game. Sorry for being such a noob. =)

Thanks a lot for the help.