If you are looking for a more in-depth guide, read Faulken's Assassin Guide.
When you reach level 25, you can become an Assassin in Prontera. The job change quest will be given to you wherever you are as soon as you hit the level requirement, so you will not need any guides or whatsoever for this.
Unlike RO1, you can carry skill points from your thief to your assassin. I advice any assassin to have 7 SP saved up when you job change, so you can instantly level your shadow form to level 5. This skill will change your looks and give you a 30% attack power and 30% movement speed boost. It is well worth saving the points for, especially for PvE purposes.
As for thief skills I recommend training skills in the following order: Throw Dagger 1, Poison Weapon 1, Double Attack 5, Deadly Blow 5.
My recommended (DPS oriented) build for a lvl 25 assassin: http://ro2base.com/b...1.1.1/0.0.0.0.0
After trying AGI and STR builds, I recommend you to build STR before AGI. The actual crit and evasion rates which you can see in your stat window are heavily influenced (scaled down) by your level and thus do not scale all that well. Your attack damage however, does not and this makes STR the better investment.
I recommend 46 STR, 33 AGI and 8 VIT as your final build, and to only invest in STR while you are a thief.
Like RO1, don't worry too much about cards at the beginning: just stick whatever good STR / AGI cards you can find in your slots.
If you're playing with friends, split professions between you. If you're solo, I recommend alchemist.
Edited by NLJarin, 14 April 2013 - 06:26 AM.