for this guide i recommend you using auctioneer addon.
first thing you need is a budget. by starting small you can build your budget higher and higher and ending up buying motorcycles or battered hilts to resell!
start of small, go on the search page and browse items, recommended is resale and disenchant or enchanting material. now you COULD use vendor but it isn't reliable for it usually require bids which is close to the vendor price, but you should look there as well for you never know.
now this might be a bit wavy if you're unlucky but even if you do lose gold its no problem you can just use my farming tips instead if this isn't for you.
disenchant: this require an enchanter of course but it's usually easy to find enchanters on servers which will gladly d/e items for a tip, i usually tip 2-3g for each item. for this to really work you need to know your servers economy on materials but auctioneer usually got that covered i made around 300g in 20 min of effort on my server, now i was very lucky and it might not work as i said before.
enchanting materials: this is just buying mats cheap and selling for a bit higher, the addon will show you what you should sell it for to make x numbers of profit. this is a good way but not the best.
resell: behold moneymaking! now that you got a very high budget its time to resell stuff! really good items are: food, flasks, enchanting scrolls, glyphs and boe epics. note, always resell t9+ boe epics for lower usually wont sell. i recommend you to buy and sell flasks pots and foods (fish feast sell best) for the starting profit, after that move on to buy the epics such as: merlin's robe, deathfrost boots, leggings of woven death etc. after a long while you will earn much as 10k a day if you play it good.