That depends somewhat, (a) on whether you have a membership or are playing free-to-play; (b) on whether you are more interested in getting more mining experience, or in the money.
Assuming you are free-to-play, the best option for increasing mining experience is drop-mining iron. The Al Kharid mine is a good spot. Drop-mining (a.k.a. powermining) means to drop everything you mine, for faster experience.
For money, you should get about the same amount of money whether you mine clay, iron, silver, or gold. Be sure to use teleports to save some time. The best place for silver and gold is the Crafting Guild, but you need Crafting level 40 (and a brown apron) to enter.
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At your level, I would say killing Experiments ( Level 25 - 100Hp = 400exp per kill ) near Canfis.
The Bandit camp mining site, which contains 8 locations. This is the most that can be found in one area on F2P worlds.
mining at about level 14
In my opinion it is a mithril scimitar.
here's the best way to get a high level runescape acount. make an acount train all skills to 70 then train bandits till you have 99 in every combat skill. now you have a high level runescape acount :)