In addition to an Internet router which you probably already have, these are the components you'll need to buy from a vendor, install and manage:  

Gateways 

A gateway is a small radio with an antenna that receives data from sensors and allows it to be uploaded to the Internet; there are three types:

Monnit USB Gateway

Monnit USB Gateway

USB--plugged into a USB port on a computer; the computer may be connected via wire to your main Internet router or via Wi-Fi; it must be on all the time.

Mojyle iMonKit

Mojyle iMonKit

WiFi Gateway-a very small computer containing a WiFi receiver/transmitter that which communicates with your router just like a mobile device does.

Monnit Ethernet Gateway

Monnit Ethernet Gateway

Ethernet Gateway--a small box with an antenna that connects to your main Internet router using a network cable or via Wi-Fi.

    Monnit Cellular Gateway

    Monnit Cellular Gateway

    Cellular Gateway--a small box with an antenna that acts like a cell phone and connects to the Internet via cell phone towers; you must acquire a SIM card from your carrier and have a data plan (the cheapest possible as very little data is transmitted).   These are used when your fields or facilities are too far from your farm Internet router

    Sensors

    These small devices are usually battery powered but some can be plugged into a electric outlet; they include an antenna which must be able to send radio signals to a nearby gateway directly or via a repeater.

    Monnit Temperature

    Monnit Temperature

    NHR Temp/Humidity

    NHR Temp/Humidity

    Rainwise Weather Station

    Rainwise Weather Station

    Routers

    These are basically just radios with antennas that are on all the time and relay messages in the network between sensors, gateways and other routers.  Also called repeaters. 

    Solar/Wind power

    Routers and cellular gateways must be powered all the time, so if AC power is not available for router locations you will need deep cycle batteries charged by solar panels, wind turbines or both; if you regularly charge 12 volt batteries for cattle or deer fences you may use those.

    Mounting gear

    Most farms have the tools, hardware, scrap wood, poles and other items that will be needed to mount the components in positions that allow the sensors to be in the correct position but allow their signals to connect with routers and gateways.  Most vendors do not supply such mounting solutions especially for in the field because the situations can be so varied.

    Portal

    Your gateway will upload your sensor readings into a database in the "cloud" account maintained by your vendor.  Your vendor provides a web browser interface as well as mobile device apps to access and display the readings and to send you alerts via email or text message when a value exceeds a limit.

    Internet Router

    If your gateway is plugged into a port on your Internet Router current router may be fine.  But if you're using WiFi gateways or you and your crew use WiFi a lot anyway you may want to consider upgrading to the latest wireless-N, dual band type router for under $100.  Here's a small tutorial that explains why:

    802.11n is faster than 802.11g, which itself is faster than the earlier 802.11b. 802.11n offers better performance, more range, and improved reliability.

    802.11n adds technology called multiple-input multiple-output (MIMO), a signal processing and smart antenna technique for transmitting multiple data streams through multiple antennas. MIMO provides Up to five times the performance and up to twice the range compared to the earlier 802.11g standard.

    Standard Speed Range Frequency
    802.11b 11 Mbps 150 Feet 2.4 GHz
    802.11g 54 Mbps 50 Feet 2.4 GHz
    802.11n 300-450 Mbps 175 Feet 2.4/5 GHz

    Newer Dual Band Wireless Routers have the capability to transmit on the 5 GHz and 2.4 GHz wireless band.

    2.4GHz though more widespread in usage (all 802.11b and g devices run on 2.4GHz only) has only 3 non-overlapping channels for transmission, which are crowded due to a lot of interfering devices- other Wi-Fi access points, microwave ovens, cordless phones, Bluetooth devices, baby monitors, etc. all make for a noisy environment which increase interference and degrade the performance.

    5GHz channel is much cleaner with less interference with 23 non-overlapping channels- 8 times more than 2.4GHz for transmission, which makes it suitable for applications like Video streaming and Gaming which are very sensitive to packet loss and delays.

    Also several newer devices tend to use 5GHz band if available, this leaves the 2.4 free from clutter and therefore can be used more effectively for low bandwidth sensor monitoring applications.