You can use a stand alone meter or the one built into your radio if it has one. Many radios also have an SWR warning light that will warn you if your SWR reaches critical level that may damage the finals in the radio. Dont rely on this, but if you ever see it light up, check your SWR, coax, antenna, grounds, connections etc.
The objective behind tuning your antenna is to make these two readings as close as possible. Getting down to a 1.5:1 ratio or below makes for a passable broadcast signal. There are two basic points to understand before adjusting the length of your antenna:
If the SWR on channel 40 is higher than that on channel 1, your antenna is too long.
If the SWR on channel 1 is higher than that on channel 40, your antenna is too short.
from this site
CB Antenna Tuning Instructions | Right Channel Radios
That is the basics of the tuning process, if you are new at this, getting something with a set screw for adjustment or a threaded tip like a firestik would be a good idea as opposed to one that you physically have to cut, cut off a bit too much and your hooped.
Make sure the antenna is properly grounded and ignore any of the things you read on the internet about 18ft coax being a magic length. Use the shortest length necessary to get from the radio to the antenna, a good RG8X would be a good choice, avoid RG58 if you can. If everything is terminated properly and the antenna is grounded properly, your coax length will have no effect on the SWR. If length affects SWR, something is not right in the installation. Seal up your exterior connection (unless you buy an antenna with a hermetically sealed junction in the base) to keep out moisture and prevent corrosion. Some like Silicon or RTV, I have used both, but I prefer the 3M rubber splicing tape. Easy to use and unlike silicon, it can be removed by gently slicing it with a box cutter and peeling it away in case you ever have to repair the connection.
As for power to the radio, if this is mobile, many people suggest running directly to the battery. This is ideal, however, 90% of the installs I have done, I have use the fuse taps, AKA "add a circuit" to piggy back off of a circuit such as the cars radio to allow the CB to shut off with the ignition. I have had great results doing this, but, keep in mind every vehicle is different.
Good luck
73s
cheers