I have a rogue (level 8) and I would like to make him into an assassin, what do I do and what do I concentrate on to achieve this

If you're focusing on assassination then you'll be making a back-row melee rogue. For a melee rogue your only real weapon choice (at least if you want any bonuses) is daggers, so you'll basically be dumping points into daggers and assassination (you won't have points left for anything else if you want to max one of these and get the other up to a high level). There are other considerations during character creation, but as you've already started your game I won't go into these.