May seem like an easy question but its not. There are many discussion threads here about this topic.
Things like "Present as small a profile as possible" basically point your bow at the source of the active sonar. Less sonar return off a smaller target as opposed bouncing off the sides of your boat... a larger target.
Run quiet... Slow the boat and rig for silent running (its a button on the bottom of the screen.
Run deep... Dive untill you hear your WO say "Passing thermal layer" you can sort of hide below that.
These are three very basic concepts but the best way to avoid detection is to not let them detect you
By that I mean try to not let them know you are there in the first place. Once they know your out there your going to be hunted relentlessly.
One trick the pickets are fond of is working in pairs... One driving ahead at speed pinging away while his buddy drifts quietly listening. Try to keep situational awarness, who is where doing what.
Its not easy but hope this helps some. Search the forums Im sure you will find further more helpfull info.
Oh almost forgot.... Welcome to SubSim