Commodities market is where you buy goods, travel to different system and sell those goods there for profit.
Back in first Elite at 1984 we used paper and pencil to write down market prices around the star systems, now in Elite Dangerous at 2014 we have 3rd party tools and web sites to aid our trading so we dont have to write anything down on paper.
Galaxy map's trading routes are really not useful for trading, you should use 3rd party tools and web sites instead.
What you need is cargo ship equipped with cargo racks and some credits. It all depends on which lever you are starting your trading, like brand new Commander will get more credits from doing bulletin board missions or bounty hunting than trading in sidewinder. However if you must you can start trading in sidewinder too, its just very slow progress. Basically ships like Hauler and Cobra are the ones where you start to make some better profits.
In Elite v1.4.01 the best trading ship is Lakon Type-9 without shields which gets you 532 tons of cargo space on 12.70Ly jump range.
So how do you trade? Well if you are sidewinder beginner Commander you should start with EDDB website which works nicely in browser (obviously) and is easy to use. When you are more advanced in Elite and trading, you have only one tool to turn to and that is TradeDangerous which is the best trading tool available... the catch is, it uses Python and is command line so unless you know little bit about computers you might have difficulties using it.
Most important thing with low credit balance trading is to always keep your ships Rebuy cost credits before departing from station. Especially if you are beginner and been trading several hours to get into that hauler/cobra etc ship, you somehow get killed, lose all your credits and ship... is very bad, trust me on this; do not depart without rebuy credits.
Below is some misc notes and info, not really trading basic info at all but no other place where to put this heh. If you are beginner trader, you can absolutely ignore everything below here.
Trading runs in Lakon Type-9 cargo ship in perfect conditions (no toilet break, no telephone, no food, no family, no nothing but 100% Elite concentration), how long it takes to depart, hyper jump and dock at destination. These are my test results.
Departure from station, 4 jumps, arrival to station: 7min 15sec and 7min 48sec
Departure from station, 2 jumps, arrival to station: 4min 25sec
Buy commoditys, depart, 2 jumps, arrival, sell commoditys: 5min 12sec and 6min 18sec
You do have to realize that this is just few trade runs which were timed. It varies so much of what kind of other ship traffic you get when docking and the dropping out of hyper space near the star location and your next jump destination etc. So many variables. However in these timings there were no real life (as listed above) distractions.
All in all its stupid to list X per hour stuff in elite as so many variables affect it. Just enjoy the game and dont worry what you can manage to do in an hour.
Un-docking timing from moment of clicking launch to starting charge FSD.
Station depart cheating: 31sec
Station depart normal: 55sec, 2 x 58sec, 2 x 1min 1sec
Un-docking cheating means that you select Launch from the ship menu, then immediately hit ESC, then exit to main menu and then start to join back into game. Now you appear about 20km outside the station without mass lock and ready to hyper jump. Some people use this as cheat to bypass the (in their opinion, difficult) un-docking procedures.
The results are that cheating method is about 30 seconds quicker on ideal conditions (no traffic in docking corridor), however its extremely lame and gamey cheating method and in big picture its not worth the time saved.
Buy imperial slaves at a discount in Torvals Space. Take them about 2 or 3 jumps into Delaines space where black markets have a buff and sell them. Easily 3900 - 4100 credit per ton profit.