Super Mario Galaxy

How to say, just complete the required for these comet stars. Talk to the Purple Chiko near the first dome, for infos about the Comet Star, only have to get luck!

For goldleaf - go to the star mission with the ? on it, and get up to the big tree that's on the edge of the stage. If memory serves, it's near the bee that's pounding the ground repeatedly. Behind that tree is a bubble machine - get in the bubble and ring the bell. The rest is self explanatory.

Deep Dark #3 - go to the bottom of the lake and find guppy. Go through all of his rings to get the star. The tricky part is that there's a boost ring - that ring does NOT count as one of the rings you have to swim through. Look for the rings with numbers.

Dreadnought secret star - Go into the star with a ? on it, go through the tube right at the very beginning, then blow up all the garbage using the bombs. Aim for the yellow dots, and keep practicing. Remember that two bombs near each other will blow each other up - resulting in some faster explosions. If you really can't do it, just go on youtube for a demo - there's a lot of them there.

For the comet missions, go find the Luma near the first dome that moves comets for you. He'll give you a random comet for some star bits. Go to the level, beat the comet mission. They're all very self explanatory, just complete the missions. If you can't beat something because it's too hard, practice more. There really aren't any super secret strategies. Once again, if you really are truly stuck and frustrated, hit up youtube - videos will teach far more than anything anyone writes down here.

Good luck!