Since no one is willing to...
I'll be the first to actually give you tips on being a newbie assassin...
1. Start on adding up on STR as sins need to be able to kill monsters with as little attacks as possible. However, too much could screw you over...
2. AGI is an Assassin's primary stat since it increases Attack speed and dodge rate.
3. Two types of Assassins: Dual Dagger(DD) and Critical Katar (Crit)
4. Max Double Attack and Improve dodge as a thief (Double first then dodge)
5. Get the skill Hide to at least lvl 6 as a thief
6. Critical Assassins rely on their critical hits (which break through an enemy's defense) so adding LUK is recommended. Also, katars double the assassin's critical percentage.
7. As their name suggests Dual Dagger assassins use a pair of daggers. DD assassins rely on heavy damage input but are potentially slow without Doppelganger cards.
8. Change to assassin at job level 45 or more (recommended is job 50)
9. INT is useless, don't get it...
10. DD assassins need around 60 DEX maximum without phreonni cards. Crit sins need around 30 DEX (since critical hits never miss) However, as I stated before, crit sins need a little investment on LUK.
11. As an assassin, Cloaking is your friend in PVP or WOE, so max it.
12. Decide if you want DD or crit. Choosing both will not work. If DD, max Right Hand and Left Hand Mastery. If crit, max Katar Mastery and if you want, Sonic Blow.
13. Enchant Poison is also useful. Just make sure your opponent isn't poison property or wearing a poison property armor.
14. Poison React counters any normal attack, so you might wanna get it...
15.

16. Profit...
Assassin Crosses are a different story, they're WAY stronger than normal Assassins.
I don't wanna check over the tips, so just tell me if some thing's wrong...