Levels 1-20 should be just mining copper and tin from wherever you can find it, I usually just mine at the rocks next to the Lumbridge Castle. Drop or keep the final products, up to you.
20-60- Here you'll just be mining Iron ore over and over again, don't both with coal as it is slow xp, but if you really need the money, you can mine it here and there. My advice to you is to mine iron next to the PH portal near Port Sarim, there is a huge place there to just mine iron and I think gold also.
60-99 Should be a mix of coal and iron, mostly coal though because at this level they go pretty fast, and you should definitely be mining in the Guild behind (East?) Falador bank. You can drop the ores if you want, but since you'll be getting Mith also if you decided to mine it, it's better in the long run to bank them.