Level 15

How much nerve will i need to reach 15 from 12

It depends on the crime you’re doing, if you have enhancers, job perks, faction perks, etc. not a lot of info to go off 

If you want to get to level 15 fast, just spam attack and leave on inactive players around your level.  It gives the most XP.

Unfortunately you don't have a donator status so you're missing out on 50 energy and the faster regeneration time, but if you just log in every time you have full energy and do some attacks you'll probably be there within a couple days max. Take a couple Xanax (+250 energy) to make it faster. 

If you are only using your nerve, and not using any energy for attacks, then alot.

Your best bet is to use your energy for attacks on other players, and selecting the "leave" option. 

As for how much energy, it really depends on the amount of respect you gain from the attacks. (This is the little number at the end of the attack log)

Higher respect earned = more XP gained = level up faster

Everything you do in Torn earns you a tiny amount of XP, but attacking is by far the best and fastest. 

Appreciate the advice,

While in general res and xp are gained similarly, they're not a perfect 1:1, leaving will give you more xp than hosping (but the same respect), for example

Attack high level players (stats don't matter) and leave them for your best xp :)

{Apparently}Appreciated, about RW participation does this add experience to each assist done or do assist add experience also or not at all

Also will it be possible to gain three levels needed before the weekend; Thoughts

No, war hits won't give extra xp unless you have someone who can med out for you that's a high level you're hitting over and over :P

Only level, end type (leave, hosp, mug) and proficiency matter for level XP (and I guess there's a book and a day for it too but those are outside of your control mostly lol)

Well advised, appreciate the info

Here's a guide that summarizes how to level up fast.