Combat is the pretty much the easiest to train on ech0.
-Buy armor weapon, and food well you start with 100k mantas so you know...
*Home, use the portals, low level, mid level, and high level portals. You can also do it though slayer, look on my slayer guide to find out how.
What to do:
*The fastest way to train up combat is to use controlled attacks. You get exp for all skills, the same exp you would if you just used aggressive or accurate or defensive. So yea this is a really easy skill. Good luck, hope this guide helps