Starting Off
You will spawn in and be given your objectives. They may look something like this:
Absorb 3 compatible genomes
Assassinate xxxxx the station engineer
Steal a hand tele
Escape alive on the escape shuttle
Stings
There is a vast list of stings available. The two best ones to begin with is the paralysis sting and the silence sting. You need to be 1 tile away from your target to sting them. The silence sting will stop them from speaking and they wont get a message telling them they got stung. The paralysis sting causes them to collapse and be completely vulnerable, they will get a message when this happens its a good idea to silence sting first.
To absorb someones DNA you must grab them by the neck and press the absorb DNA button in the changeling tab.
Way to play
There are many ways to play as a changeling, the stealth way is one of them and the preferred among most players. If you are discovered and they think your a ling they are likely to ship you straight off to the crematorium to burn your body so you cant heal (if you get killed and your head is still intact you can use regenerative stasis to recover, this also works when alive to heal wounds.)
There will usually be more than one ling on the station at one time, to communicate with them use :g (this comm's cannot be heard by anyone else so even if standing beside someone they wont hear you although its best to communicate where people wouldn't hear you in case you fail)
Lings will occasionally work together to complete their objectives and back each other up in tough situations. It is often good to get a good standing with other lings so they are more likely to come rescue you if your found out or brigged, about to be cremated etc.
Found out
Sometimes it is completely out of your control and you may be found out. There are ways to get out of this should you manage to escape but have your identity compromised. The most common way is to change your appearance into one of your previous kills. It can be good to take the ID's off of their bodies when you kill them so should you have to turn into them you have ID. If you have been handcuffed or are in custody and need a quick exit, use lesser form which turns you into a monkey and crawl through a vent and escape.
Disposing of bodies
This is really important and is best to do it through maintenance or quiet areas where you would be less likely to be caught. Personally my favorite place to dispose of bodies is the virology disposal unit as it directly jettisons them into space. Any other external airlock will do as well if you plan to space them. If you are a doctor you could always use a body bag or a roller bed to make it seem as though you are transporting them to medbay or the morgue. The crematorium and the chapels mass driver will do the job as well. And if all else fails you can always stick them in a locker somewhere.