If you have nothing go to the chickens in Lumbridge. Kill chickens until you have about 1k feathers , sell them for 7-10 each. Then you have about 7k-10k. Buy feathers for 5 each, until you have used all your money. When you have used all of your money sell the feathers for 7-10 each again, do this until you have about 50k-100k.
If you have 50k buy nature runes for 250each, if you spend all 50k of nature runes you should have 200. Sell the 200 nature runes for 300gp-320gp each then you should make 10k and you will be at 60k do the same thing until your have about 100k.
When you have 100k it is a lot easier to merchant. All you have to do now is merchant coal or iron even. Buy coal for 150each with your 100k that should get your around 666. Sell the coal for 200GP+. Now you should be at 133k+. You should do that until you get 200k.
Now you need to buy steel bars for 600 each. With 200k that should get you 333. Sell those for 650 each now you will be at 216k. If you are a member make cannon balls. With 333 steel bars you can make 1332 cannon balls and sell them for 200 each. Now you should have around 266k. Personally I just merchant the coal until I have about 600k then I buy 1k steel bars and make them into 4k cannon balls and sell the cannon balls for 800k. So while you smith a lot you gain much experience.
Now that you know how to merchant just try to buy a lot of runes or ores or what ever you want at a low price and then sell them for more.
There you go, now you no how to merchant!
Good suggestions,
Onflame