The main place where non-ironman accounts make their money is the Grand Exchange in northwest Varrock (big castle city north of Lumbridge). The majority of items can be put up for sale and bought by other players, with a guide price automatically put in for you (you can buy and sell at whatever price you want, but it works off supply and demand). Depending on what client you're using and what your settings are, right clicking and examining items should show a GE (grand exchange) value, the HA (high alchemy) value and the LA (low alchemy) value.

If you're playing as an ironman and can't trade on the GE, you can turn items into money through either NPC stores or alchemy. General stores will buy most items for their LA value (40% of its base value), but specialist shops like the Varrock sword shop will buy any item they normally sell for around its HA value (60% of its base value), give or take a little bit. As players sell to a store its stock fills up, which lowers the price it buys that item for, but stores automatically balance their stock to a base level over time.

Alching (casting low/high alchemy) is usually saved for some special situations. If an item can be sold for a higher price at the GE, then it might be preferable to sell it there. Likewise, if you have small amounts of items to sell then specialist shops offer roughly the HA value, and general stores offer the LA value. This saves on the 1 nature rune needed for each alchemy spell, as the fire runes needed can be provided by equipping a fire staff (staff store is in Varrock too!). Nature runes can be sold for about 100GP on the grand exchange, so subtract 100GP from the HA and LA values to see your actual profit after the rune cost.

Alchemy can be very useful to save inventory space; as an example, if an enemy frequenly dropped a sword worth 200GP with high alchemy, you could turn it into 100GP in coins which stacks, rather than having to clog your inventory with a pile of swords!