As a beginning spinnaker user, avoid trying to fly the spinnaker in heavy air, obviously, but also avoid trying it in extremely light conditions. Keeping a chute flying in the lightest conditions is a challenge for even an experienced crew, and trying to do that as a beginner will simply frustrate you.
With a nice 8-10 knots of breeze, try your first hoist on a comfortable broad reach. You can hoist from the pulpit or, as we have taken to, hoist from the leeward rail midway between the bow and mast. Be sure your halyards, sheets and pole lines are all run free and not crossed over with others, lifelines, pulpit rails etc. I'd suggest you practice hoists and takedowns on the same tack before trying to gybe, that way you can quickly douse the sail if things go wrong later.
As mentioned, have plenty of sea room initially, esp for your first attempts to gybe. Ultimately the driver has the most influence on how the gybe goes, for two reasons... first he should keep the boat "under" the sail, and secondly from his/her vantage point any potential snags and problems should be immediately visible, so you can coach the crew past the snag.
Make sure you talk all aspects through before attempting the gybe (and not just the first time either)
The spinnaker can be a very rewarding sail to use, but seems always poised to catch the careless and unaware at the worst time. This is why taking the sail down should be second nature - when it has to happen it has to happen cleanly and quickly.
As far as sheets in the prop, this usually occurs when the sail has been stowed and the motor started up after the finish or heading into port. If the sheets are not all "stowed/secured" too that's when the engine may suddenly stop