Level 1-10: Stick with your newbie clothing. Do not buy anything that will be of no use. Do not even touch potions because you are still a beginner and you will not lose exp (Maple Story mesos) even if you die. Keep your sword/axe/club until you do Biggs quest. Level 11-20: When you first become a thief, you will be given a Beginner's claw which is really an untradeable Garnier, an untradeable Fruit Knife, and 3 sets of non-rechargeable Beginner's Subi throwing stars.
Level 20
Level (Mesos) 30: Now you are an official assassin. Start buying mana elixirs as a replacement of blue pot. Also, by doing a quest that requires you to kill 99 Evil Eyes give you 100 free mana elixirs. Use them when needed. Stick with white pots for now. Train, and get mesos. If you have Mokbis, which you should, sell them off and buy Kumbis Icicles or Tobis if you have the money. You should have at least 2400-2500 total stars, which are 3-5 sets of whatever star you choose. Also, get all of the level 30 gear and do the Nella Kerning City's request 2 to get your level 30 shoes.