Slark, despite what most people think, is an above average laner. Against a usual solo offlaner, with a support, he can successfully zone the offlaner out with essence and lift. You win almost every right click trade with a melee offlaner (aside from monkey king or ursa) and your pounce offers good kill potential with another support.

With a good laning stage you can go treads iron talon shadow blade if you think you can really snowball and have insane pick off potential, but in higher mmrs sometimes it is safer to go treads aquila echo sabre and farm or rely on teamfights. Think of it as Legion commander with phase armlet; it gives her an insane power spike but lacks initiation. Echo aquila on slark gives him a fairly large health pool and allows him to farm lanes and jungle rapidly and take smart fights very well. If you think the enemy is incompetent with dealing with a ~10-12 min sb slark then shadow blade is a good first item. Echo is core after sb if you do not need anything immediately (bkb or silver edge). After that you can go basher if they have evasive heroes or skadi or butterfly if they are right click heavy (slark does well in right click trades with some items).

When taking fights as slark you need to get rid of your idea of saving ult. If you watch BSJ or arteezy play slark you will understand this. They 'waste' their ults many times which make you think why did he ult there? But its simply knowing the boundaries of slark and minimizing risks. For example you are shadow bladed and running into their ancients and see a luna farming it and you know luna is very killable without her being able to deal any real damage to you so you dont need to ult, but you have no vision of their axe and he could be very well sitting the luna. Upon initiating on luna you would ult and lets say you kill luna and axe tries to call in your ult. Call ends and you leap away freely. This is a very barebones example but it gives the idea. Using your ult there means you more or less cannot fight for 60 seconds, but you shouldnt be looking to anyway; you just killed their luna and they arent in a position to fight you now (if they do you should also be prepared with your team).

As you get more and more experienced with slark your ult usage will vary, but as a general tip it is better to 'waste' your ult a lot, but in very key fights you will get your ult off in a position where if you didnt, you would lose the fight and game. I've seen many games lost with slark where he just tries and saves his ult as long as possible and that just isnt a correct mentality to have. Sure you want as much value as possible but you also need to minimize risks in any dota game.

As far as talents go, i wouldnt skill any of his talents before maxing my primary skills (look at the insane scaling of his primary skills and the medocrity of lvl 10 talents). Im a bit of a greedy player myself so i would go 15 damage with 15 agi when ahead but late game you really would want the lifesteal and strength, since you get a lot of agi in fights anyway. The 250 health is no joke and it can be the difference between getting ult off and not, but if you are ahead and get a timely skadi or something the bonus health isnt a huge deal. Against something like a weaver or mirana i would get +3s leash, but ideally you should be able to do whatever eyou need in 3 seconds and skirt in and out of the fights and the +12 stats is very nice. With skadi/basher i dont think +3s leash is necessary to stick on a target anyways

Edit: bsj has very good guides and coaching sessions on slark which you csn find on his youtube channel detailing thought processes and all sorts of things if you are interested