Eddie, the main difference between FW and SW is the level of time required for maintenance and the amount of money. You really should get a book, but here are some of the basic things you will need:
FW:
*Tank
*Standard light (usually included and either incandescent or NO flourescent)
*Heater
*Filter
*Substrate (like gravel)
*Livestock $2 - $15 for most FW fish
SW:
*Tank
*New Lighting (if you want to keep anything other than fish you will either need Power Compact flourescent lighting (PC), Very High Output flourescent lighting (VHO), or Metal Halide lighting (MH), for a 29 Gallon, you are looking at $100 - $300)
*Heater
*Protein Skimmer (instead of filter, although some people use both a skimmer and a filter $80 - $200+)
*Substrate (either Live Sand or Crushed Coral with LS being preferred by most of the people here)
*Live Rock 1 - 2 lbs per gallon usually $4 - $6 per pound pre cured if you find a good deal. This can get pretty expensive
*Water, I get mine pre mixed at the LFS, but water quality is more of an issue with SW than FW, so you really shouldn't use tap water like you can for a FW tank. Even if you do have a fish only SW tank and use tap water, you will still need to buy salt from the LFS which is another added expense.
*Test kit - I have never had to test my FW tank, but I have to test my SW tank regularly and add some chemical additives from time to time
*Livestock - $8 - $crazymoney (although in a 29G probably more like $8 - $30 per fish. If you want corals, they can get pretty pricey)
Depending on what type of SW tank you set up, you will probably spend at least $200 - $500 more than if you made your 29 a FW tank. You will also need patience. With FW you can get your fish the same day you set up the tank. With SW you will set up the tank and have it sitting with just water and Live Rock for about a month before it is ready for fish. Another difference is the amount of fish you can keep. I have a 30G that I switched from FW to SW, and when it was FW I had 15 - 20 small - medium sized fish in it with no problems. Now that it is SW, I only have 3 small SW fish, and I can't see having any more than 5 - 6 small SW fish. Also, because a 29 is so small, you won't be able to keep a lot of fish you may be interested in, no porcupine puffers, no Dory's, no Yellow tangs, no Angel fish (except maybe a dwarf Angel), sadly the list of fish you can't keep in your tank is longer than the list of fish you can keep.
Don't think I'm trying to discourage you, just letting you know what to expect. Before making the switch I read "The New Marine Aquarium" by Michael Paletta and it helped me a lot. And despite the extra money and effort, I am much happier with my SW tank than I was when it was FW.
