Viper is one of my favorite heroes to play. He can be played many ways, but my guide will outline my favorite way to play him.
Viper is a very powerful hero from pretty early in the game. Ideally, you can solo the long lane. Viper's Poison Attack is great for harassing, and his attack animation makes getting last hits easy. He's very survivable with decent map awareness and his range will allow CS against 2 heroes.
Soloing with a hero who has a stun will guarantee kills early on. Nethertoxin will help get early kills. You're poison will whittle enemies down and Nethertoxin will boost your damage. Anyone who tries to tower dive you will die. Viper's poison will wear them down and slow them, with the tower helping out as well.
A quick wraith band will also help with CS. I like to throw a point in Corrosive Skin early on since it helps a bit with escapes. Power Treads on strength will make you less of a glass cannon. Phase boots aren't a bad idea either, but might leave you a bit too squishy.
A 20 minute mantle should be your goal. The split will help avoid some stuns that are thrown Viper's way, and the stat boost is great. The splitting is useful, but better later on. CS-ing should be easy now, with Viper doing around 110-125 damage without his poison. You can enter team fights. Using Viper Strike is an awesome ulti with a low cool down. Whomever you hit in a team fight will die within a few hits after you hit them with VS. With a Butterfly Viper is almost OP. Damage will be very high. Adding a heart makes him almost unkillable. Spamming your poison in team fights makes them fly by.
At this point, you are a beast. Use Viper Strike to hit the enemy with the highest HP and watch it melt away. With a scepter, Viper Strike's cool down is almost nothing. Late game Viper is one of the best carries in the game. Viper is VERY dependent on farm. With moderate CS he is strong. As I said before, a 20-25 minute manta is ideal and will go a long way towards your end game.
Good Luck and Have Fun!!