U can start it at lvl 40 the quest is called enchance your weapon. You can start it in giran grand master jurek located in mage guild, oren magister gaiden de guild and finally in aden magister winonin think its the temple or some similar place. Look here (http://lineage2.stratics.com/content/features/sa/) for which mobs who can lvl and such, well u need to be lvl 55+ to be able to safely lvl them to any useful lvl, hell can prolly be lvl 45 to get them to stage 5, but low c grade aint that useful in my world.
U might also wanna consider the option of buying the SC, im guessing u going with dark screamer and that requires a lvl 7 crystal should cost around 500k til 1 mil. Best option would be to join a decent clan and either get help with the funds or the lvling of the SC.