I'll help you out here man; but heads up i'm a marines player and everything I know about orks is a brief liason a few months back and regular opponents...
As far as legality is concerned, you have the core of the army .I believe the painboy is an elite choice; so you'll need to separate out your big mek meganob. the big mek and warboss are your 2 compulsory HQ choices to fill out a battalion.
Stick 1 Nob and 9 boyz together. you've got a minimum size troop unit.
repeat above x2, and you've got a battalion detatchment, unlocking you 5CPs in addition to your 3CPs for being battleforged (Battle-forged just means that all of your models in the list fit into detatchments).
from there you can add in your 6 warbikes, 5 burnas and 5 tankbustas… painboy if you want to.. note that with the big mek absent, your 2 meganobs are unusable....
...That is unless you pay 1CP for an auxiliary support detachment, which may feature a single understrength unit. simply pay for your 2 meganobs and their wargear... note that this unit of 2 will not benefit from your clan kultur
Will it be competitive? Nope. Will many armies utterly stomp it? Yep. But its legal, and gets some minis on the board while you learn the game. try asking for fluffy opponents lists, or suggest people try using units they avoid in competitive games to see how they perform.... once you have a feel forthe game and your personal play style; then I might worry abut expanding... probably into some more (20) boyz, a weirdboy and maybe some armour?
hope this helps,