With Shen solo top you will need much life regen and a bit armor, don´t play to aggressive let the enemy top laner push and poke him with Q or attack with E, W, Q combo

Early Warmogs with Shen is just op, you don´t have to go back, you tank everything and you can stay for hours at the top lane

Buy Sunfire, if you taunt your enemy with E and attack him with the W, Q combo your passiv will deal much damage and sunfire does the rest

If you want to deal damage buy Frozen Mallet, it gives you life and damage, after Frozen Mallet go for Randui´s it´s very usefull in teamfights.
Now you have the choice, buy Atmas for more damage, or buy Runic Bulwark

use your passive for farming minions, creeps and use a Q, passive combo for farming too
you will easy get 100 farm in less than 10 minutes which is probably pretty much for an AD off tank/Tank

In teamfight or fights between 2 or more players pay attention that you always taunt the enemy damage dealer and quickly activate your shield and use your Q, with Sunfire you deal tons of damage in 1vs1.
Sehn is a Champ that you play agressive, but be sure that you don´t push your lane ! let the enemy top laner push and attack him often with your E, W, Q combo