This is just a quick build I made, not going to be a full guide, but just the build and a couple of words with you. Kassadin is a great character, but sucks early game, and you resort to last hitting creeps with your Q as you're melee and you face ranged people in solo lanes. Once you hit level 6, use your R + E + Q combo and harass, and level 12 is when you start becoming powerful. Note, when you're not 1v1ing, your role in team fights is to silence casters, riftwalk on as many people possible, as well as spam your E whenever you can. Make sure you are not focused, this way you are not taken out easily.

These items on the build are just the core main thing, but, of course there are situational items as well.

Such as:
Mejai's Soulstealer (If you're confident that you can own the other team)
Abyssal Scepter (If you have a caster on the enemy team, like Karthus)
Void Staff (If they are building MR)
Haunting Guise (If you need a little more health and AP, as well as the Magic Pen)
Banshee's Veil (If the enemy team is full of casters like Veigar, Karthus, Brand)

That's really about it. I just made this quickly and I will edit it more to become a full guide depending on the feedback. Good luck, have fun!