50 so you can start using teleports, provided you have the magic level. This was taken from another website, some-one wrote it in 2007 and i thought it would be useful
"To start I recommend getting all these items now; -200k-300k -a bucket -pickaxe(any) -watering can -5-8 plant 1's -a spade -a hammer -a saw ~~1-3~~ First go to a real-estate agent and buy a house in rimmington for 5k. Now walk to rimmington and go to your house(building mode). Now plant your plants in the correct spot by digging them up after planting them and re-planting. You now have atleast 3 contruction. ~~3-30~~ Drop your spade, and watering can. Now go to the mining spot just near the house portal. Weild your pickaxe and mine a full inventory of clay. Now walk back to the portal and walk a little to the fountain. Use your bucket to make all soft clay. Go back into your house, building mode, and make a bunch of fireplaces in your living room with your soft clay. You can repeat this all the way to 99 if your really into it. Why? Because its free powerbuilding. I suggest about level 30 only. Alternate: Rather than mining your clay and wetting it you can sell it to the shop and rebuy it, be careful other people can steal it. This costs 150 or so more per soft clay though so if your rich its good. ~~30-50~~ Now buy all the oak planks you can with 200k-300k and make oak bookcases over, and over, and over. This requires level 29 construction to make and they give 180xp each time. Since you have 30 constuction you might want to consider buying a "cook" as she can carry 16 oak planks at-a-time for you. CONGRATS! You now have 50 construction! "Why 50?" you ask. Well 2 reasons 50 is the level you get a portal room with kharyll teleport. Which means you can pick fungus if you completed 'The Nature Spirit' quest. Also 50 is kindof the middle level in RS.
HAVE FUN!"
Please read post below that contains vital extra information as this is outdated due to a 2007 having a different runescape economy.