In team fights you don't really want to initiate, but you want to sort of...duo initiate with someone else. If you initiate and your team isn't on skype with you 9 times out of 10 you'll get focused and killed before your team deals enough damage to make your death worth it. You want to be fearless and have as much battle time under your belt as possible.

1v1, you use your basic skill combo for harass, if you don't land your first E then your doing it wrong. Your ult is mainly used for tower diving and tower diving bait, watch for ignites.

If you get exhausted while attacking another melee, walk on top of them, E away + walk away tll exhaust ends, E back to them.

Thats all you really need to know about renekton in a nutshell.

Normal whatever runes, you might want some MR per level glyphs instead

You like doran blades, get two of them
You like merc treads cuz it sucks to be CC'd in a team fight
Bilge water cutlass synergizes so well with renekton its crazy
Brutalizer cuz u want armor pen
Frozen mallet for that HP + a nice CC to make your more of a threat
atmas is a must have for renekton due to his ult + high amounts of HP with this build
Aeges of legion because if your this far into the game and your team still has not won they could really use a little help in the stat area.