Shen is not strong before level 3. Strongest tactic is to do full 5 camp clear. Shen reaches his powerspike at level 3 because he can access his W.

After reaching first powerspike you can gank any lane you'd prefer (I recommend to kill the weakest ones for free kills and gold --> botlaners), top lane is usually a quite free target due to your advantages and W.

When Shen reaches level 6, he has his ultimate (teleport) to his access. This won't benefit you alot but late game it will be very powerful due to fast movement and good map control. Teleport will be very catchy and good when stealing or going for objectives (drake, baron etc.) Shen's ultimate is also capable of saving allies because of its' sheilding. The shield will become near almost 1000 so it will be very handy from time to time.

After reaching mid game and getting your Kraken Slayer, the fun will begin. Shen has huge potential even when he has only 1 item. The damage is just crazy with his Q. Shen is a good duelist due to his good passive and W blocking ability.

When you have your first item and boots you need to start map controlling. Shen's E ability can dash trough units and walls so it's a great style to move in jungle. Ganking opportunities will be amazing and powerful (also you will have a lot of ganking possibilities) Just gank as many times as possible and find good gaps to do your jungle clears :)

At the end game Shen will nearly be immortal if you make good plays and dont int. Due to your runes you will have a lot of life steal due to your items and runes. High life steal will be a good counterpart to your low max HP.

When you're playing Shen, don't roam at late game alone, stick with the team or atleast have someone near you.