Paranormal investigators have a variety of tools to use, from the simplest tape recorder so infrared cameras.
The first item I'm going to talk about the camera. This can be a cheap disposable camera to a expensive 35mm to any digital camera. There are also the infrared cameras. These are nice to have, but can get costly. Having an expensive camera doesn't mean you have the best camera. There is not one better than another.
Other helpful tool is voice recorder. This is used to record EVP's, electronic voice phenomenon. This is another tool that you don't have to spend a lot of money on. You can use a tape recorder with a microphone, a micro tape recorder, or a digital recorder.
EMF, electromagnetic field, is used to find electromagnetic fields. In theory paranormal activity can be detected using an EMF if the meter reads a disturbance. You will need to take into consideration that they can also pick up on home appliances and cell phones as well as power lines. Still a useful tool.
Thermometers are very useful. I don't mean the kind you stick under your tongue. Sudden drops or spikes in temperature can also indicate paranormal activity. There are affordable ones as well as expensive ones. As before just because it's expensive doesn't always make it better.
Infrared Motion Detectors are good tools to have but are not necessary. They detect movement. This tool is used by setting it up in a room when a investigator will not be in the room. You can pick up these starting at a reasonable price.
And of course don't forget you flashlight and lots of extra batteries. Paranormal activity can and will a lot of times drain your batteries almost instantly. You will also need notebook paper and pens for interviews and notes. A laptop computer is also another good tool to have.