Try to find a bag that fits your kettle so that the grain can come in contact with all the brewing liquor. Mash for 75 minutes. I use tastybrew.com 's calculator to figure out my strike water temps. After mashing, lift the bag off the bottom and raise the temperature of the wort to 175 degrees (higher than 170, but when you lower the bag it will drop in temp because of the cooler grain), lower the bag and stir gently but well for a minute or so. This helps rinse more sugars of the grains. A dunk sparge in a seperate kettle helps, but may be adding another layer of complexity for your first batch, which should be easy.
Use Whirlfloc 10 to 15 minutes from the end of boil. It will really help settle out proteins.
Don't get upset if the numbers you get are a little different from what you set out for. (1.050 instead of 1.055).
Have fun with it!