It's a fun however, not a very lucrative skill. Smithing is basically when you transform bars into objects. Selling the bars RS gold is better than making something with them. Five rune bars are needed to make a runeplatebody. Each bar will cost approximately 17k. You will spend 85k on the platebody if you multiply 17x5 = 85. The bad news is that a Rune Platebody can cost about 60k. This basically means you've just lost 25k. If you don't wish to lose money, do not make a forge.
Do you think that forging was the only thing that can be done with Smithing? But it's not! There is also a part called Smelting. Smelting is the process that transform ores into bars and the most appealing aspect is that it's hugely profitable. To smelt, for instance, a steel bar you need 1 iron and 2 coal ores. Iron is 100 coins, and coal is 178 coins. You would need to mine 1 iron and two coals to make 1 bar. However, if you were selling ores, you'd get cheap RuneScape gold 456 coins. If you could melt them, you'd receive about 163 coins extra. Steel bars are 619 coins.