Hey, i'm a new player here and made a rogue and currently am level 18. Should i worry about Hp washing? Also i want to become an assassin how should I allocate my ap points in luk or dex?
I'm no expert on thieves but: Short answer: Yes you need HP washing to become the very best. No you don't need to HP wash to just Zak and grind and Horntail at later levels (I'd guess 16x+ ish?) Also, the general consensus I've heard is you should stay dexless until you get a good craven at which point you turn into a low dex thief. Capping around 60-80 DEX is the average AFAIK.
Take a quick look at this guide by @Sila and me, there's info about gear and how to distribute your AP there. As to HP washing, it really depends on your goals. Without any HP washing, you're going to have a hard time. Since this is your first character, washing too much isn't the way to go either; I recommend going for just 30-40 base INT, this will cost you around 1b (which won't be much over the course of a month or more, you can get 8,000 NX a day) if you buy all AP resets with mesos. This'll get you to 5.6k HP, which is required for HT, around LV. 12x-13x. You can wash more, but it's not 100% needed; would be a nice addition, of course. TL; DR: Read the guide for some tips and tricks and you don't have to wash too much; 30-40 base INT is more than enough and isn't expensive.