This thread here is the preferred place to ask any sort of general modding questions you may have. This keeps them all in one place and the number of redundant threads down. If you want to make a development thread to talk about ideas for your mod, you may do so
here but those aren't really for asking technical questions.
Anyway I'm going to write a script now off the top of my head and it's going to be full of errors most likely
<entity 200 would be the chest(closed) sprite>
<it needs flag 0x2000 to run its event when the player interacts with it>
#0200
<KEY<FLJ0100:0001<FL+0100<FMU<CNP0200:0021:0000<MSGYou open the chest.<NOD<CMU0016<CLR
<GIT0002<AM+0002:0000You obtained the =Polar Star=!<WAI0163<NOD<RMU
<GIT0000<CLRIt's a pretty sweet gun.<NOD<END
Additionally the chest entity should have the "Disappear when flagID set" flag checked, and have its flag ID set to the value FL+'d in the script (0100 in this case). Above the chest entity should be an entity 0021 (chest closed) with the "Appear when flagID set" and "Option 2" and "Run script when PC press down" flags one square above the closed chest, with the same flagID as the chest and event #0001 (the global empty event)
For the rest of the commands I don't know, they should be pretty self-explainatory if you just read the description in CE but then again I'm not the one asking the question so idunno.