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Hi Christine... Thier are no rules to this, it's all about conditions really.. I start with a 24 -25mm eyepeice to look at objects and you will get a nice clear image. Then keep trying more and more power. IE try a 15 - 17mm and see if the image stays clear. The barlow will in most cases double the power of your eyepiece. IE a 25mm would now be a 12.5mm.. You will know when you have used too much power as the image becomes unclear or fuzzy.. This will change from night to night dependent on the conditions. On a still cool winters night you wil be able to use more power...
As for nebulae and a galaxy I have found that often less power is better. You often need a wider view (less close up) to see all the Nebulosity. I oftem use a 32mm eyepiece for this purpose...
It's all a bit of trial and error to see what suits you situation best. You will get the hang of it in time.. It's all part of the fun..
Hope I have been some help, and I am sure others will give you some good advice also..
Regards
Mardy
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