Talon is very good at farming. Level up his rake(W) as fast as possible to 5. This can essentially one shot a minion wave. Many people at lower levels do not understand the importances of balancing harassing with farming. Farm up well early game, dominate late game.
If Talon is laning with a partner, choose a partner with good crowd control. Talon can easily harass with his rake while still farming. Just use it while the enemy is standing by the minions you are trying to kill. When an enemy is low enough, get the partner to stun or snare and you can cutthroat in and use to burst to bring down the enemy. Note that cutthroat will not only increase your damage but silence the enemy so that they cant ghost/flash or use one of their spells to escape before you kill them.
If talon is laning top solo, once again focus on leveling up rake and farming with it. But with talon being alone he can harass more aggressively by using cutthroat to silence the target, use your skills and run away before the silence ends. This usually results in you dealing a lot of damage to the enemy than they can retaliate with.
When to engage:
I assume you mean in a teamfight. There are two ways to play talon in teamfights. The first way is to play like a normal assassin and stay behind your team hidden. Only when the team's initiator(tank/offtank) go in and the fight has started so you go in and focus their ap carry. I say this because talon has a silence which will prevent their ap carry from doing any damage.
Another way to play him that most people are usually reluctant to do is be the initiator. This will work really well if the opposing team does not have too many stuns and/or do not have oracles. You basically cutthroat into their ap carry and immediately ult to stealth. This will cause them to immediately switch to another target(preferably your tank/bruiser who has come in by now). A benefit of this is, you get to damage everyone with the beginning of your ult AND the end of your ult.
Talon is as I've said before a burst assassin. He bursts and then runs off. So if anyone ever tells you to built attack speed on him. DON'T. Build pure AD and armor pen on him. You shouldn't have to stay and autoattack because any tanky AD DPS can kill you in a toe to toe fight
I would start with dorians blade first. At first recall, hopefully enough gold to buy a pair of boots and the brutalizer. Next built the boot into Ionian Boots for the CDR (Once again berserkers's attack speed is less desirable compared to CDR where you can burst more often), and start of the blood thirster. Then i would finish the brutalizer into Youmuu's GhostBlase and bloodthirster. Afterwards i would built last whisper (that 40% armor pen is OP!) Infinity Edge and Another Blood Thirster to finish.
When runing a champions there are two things to keep in mind. Do you normally have trouble early game and would like a early game advantage or do you do well early game and want to have runes that scale really well into late game.
If you do not do very well early game and need all the advantage you need, then build flat AD Marks. But if you do well and want runes to scale better late game, built Armor Penetration Marks.
For Seals, I would take flat mana regen(replenishment) because talon is pretty mana starved early game if he wants to farm well and still harass.
For glyphs i would take flat cool down reduction.
And for Quintessances, i would take Armour Pen.
With the new change to the mastery trees, assassins like Talon can rejoice! I won't tell you exactly which to put points into but focus on the offensive tree for assassin champions like talon who focuses on burst.
The two I would recommend is: Ignite and Flash. Flash is of course one of the most powerful summoner spells there is. This can be used both offensively and defensively. If the enemy is just out of reach of your cutthroat and you can't jump to them for the finishing blow, flash will close the gap between you. Or defensively you can use it to create a greater gap between you and your enemy OR flash over a wall. You do not understand how many times that has saved my life.
Ignite's DOT and his Q's DOT does a surprising lot of combined damage. In addition ignite will reduce the amount of healing the opponent have/gets. So if you are laning against someone with heal or a lot of life steal, ignite will make you laugh gleefully as they die because they can't heal themselves from your burst.