Pretty new myself, and I had a lot of the same questions so I hope this helps.
1. what size of tent should i buy if i want to grow 2-3 plants ?
First, I recommend starting with one plant, just so you can give it all the attention it needs and you have one plant to tend to, look at, learn about, and post more questions about. but for 2 or 3 plants it depends how you grow them. You could get away with a 4 x 4 tent thats 8 ft tall if you want to use one light and grow small, but there are people who use a 4 x 4 area for one plant. but if you want to be somewhere in the middle a 4 x 8 tent will do.
Mind you, these are my opinions, not facts. I actually dont use tents, I have a room.
2. what kind of lamp should i use led/flc..... and what W200 ? W300 ? .....
I use a bulb ballast and shield combo set up for 1000W bulbs. Metal Halide for veg, High Pressure Sodium for flower. The bigger the light, the bigger the yield, the bigger the electric bill.
Also, keep in mind that your tent will in theory be shut so the bigger the light, the more heat will be produced and trapped inside. so you will need bigger fans to keep it cooler.
3. what is a good air filter system i should buy ? depend on that size of my tent
I recommend a carbon filter setup, they are expensive, but you can go with Phresh or Black Ops carbon filters. Those are two brands that have been proven useful to my friends here in the Detroit market. they also usually have a diagram on the box or if you go to their website that says what size filter is good for what size room and how big of a fan you need to properly flow the air through the filter. There are also other measures you can take, but for two or three in a single tent that should be all you need if you buy a big enough one.
4. what other equipment i should buy ? fans ? ph lvl ? carbon filters ?
Dehumidifier, Each plant at the peak of bud can perspirate up to a gallon of water a day. I never let my humidity in my room peak over 60% and try to keep it between 45 and 50%
House fan, Even in a tent I recommend a house fan blowing on your plants. it simulates wind, so the plant naturally tries to thicken its stalks to combat the wind and be able to survive. I know people who go and grab the stalks and give them a quick shake once a day after they get big enough. the thicker the stalks, the bigger buds it will grow, because the more weight it can hold.
Carbon filter, Covered that for the most part good for the smell.
Co2. I have heard that it will help your plant produce from 10% extra yields, to 50% extra yields(probably exaggerated a bit on that claim) but, there is a bag called the ExHale bag. look it up. it is relatively inexpensive, hang it over your plants pull the seal and let it do its trick. mine was around 35.00
5. should i use regular water ?
Some people claim that you should use only reverse osmosis water because its safer cleaner and ect. I use tap water. Buy some Ph up, Ph down, an eye dropper, and a testing kit from your local grow store. most test kits have instructions. just make sure that you keep your Ph in line with the recommendations you're following, for instance I keep mine right at or just under 6.0. that's what's recommended for my medium, and nutrient setup.
I'm sorry, I know I kind of rambled on, but you had a lot of vague questions and I hope this helps you get started.
