I have a 99 Honda Accord DX?

I guess in this DX model they didn't intall AC in it & i was wondering if you could install the AC in it

Yes you can, but it is not easy. The DX was the base model and AC was optional.
The AC system has many components in it, AC evaporative core, Condenser, Condenser fan,
Compressor, Accumulator, and the long AC line, you might need to add a pulley wheel for the compressor belt.

The fact that you need to ask, I don't think this will be a job for you.
The cost is not practical, it might cost more than the value of the car. New parts will costs too much, so you'll end up getting used parts, but most of the cost will be on Labor.

Your best option is to sell the car and buy an LX or EX model. It probably costs the same but much less hassle.

That'll be fun. Good news is, the car you have will have all the room needed to install the components. The hardest part is getting the dash apart and opening all the heater and blower components to get that condenser in. Fun times my friend, fun times. Let me know how it goes.

Scrap yards can help you find original Honda stuff. Otherwise, buy a broken Honda, with same motor, as a donor car.

It's not easy. That's why I suggest a donor. Removing can walk you through installing. Also, go to ebay and get a dealers shop manual.