Special Moves
Rocket Punch
+ p
Probably Sentinel's single most used special move in the game, due to his bread-and-butter hp>RP. This can be done with either lp, which goes down between 45 degrees and 60 degrees (approximations, just demonstrating variation) or hp, which punches straight.
The rocket punch can be chained from the lp>mp>short/hard RP or from 
(hard RP)
SECONDARY NOTE: this can be used as an anti-air with Z+punch, though it's not very useful outside of air combos
Sentinel Force
+ k
Using short will summon mini Sentinels that you can control vertically with the joystick; roundhouse summons slow-moving mini Sentinels that will bomb the area just in 
of you (hold the button to cause them to bomb across the screen).
of you (hold the button to cause them to bomb across the screen).
Short Sentinel Force is pretty easy to use, although it's tricky to combo afterwards because you can't use Hyper Sentinel Force while mini-Sentinels are on screen. Roundhouse Sentinel Force is harder, and is best used with an assist that pins the opponent vertically such as Blackheart. RH also is a good way to stop crossup rushdown if they use superjumps to come in at you.
Flight Mode
+ 
+ hk
N/A
Repeat to end. You can't block in flight mode, but you are way more mobile and can call assists if they come near. This is your bread and butter. Fly and show them your boots of love or your frying pan of justice. If your Sentinel doesn't fly, you're going to get killed.
Super Moves
Plasma Storm
+ 
+ hp
Mash buttons for more damage. This is a less useful super, though if it connects, it can be very damaging. See Bread-and-Butter Combos.
Hyper Sentinel Force
+ + hk
Note: Can be comboed into and out of, especially with bread-and-butter X Rocket Punch.
X Rocket Punch.
Hard drive
+ + hp
This move is one of the more useless of the supers. The best use for this super is as a DHC (such as the hailstorm), or his his launcher xx sj.lp>sj.lk>sj.lp xx HD