Seriously, just field what you have to start. Use the Grots as shields and start marching! See how you do. Use all the cover you can! Include Nobs in your army - maybe you can convert some of the ones you just bought, or paint them differently to show them as Nobs.
If this does not work for you, you might consider a little bit of fast stuff - the worst part about fighting Shooty Armies is that you have to march all day before you can get into hand-to-hand, getting shot all the way. But when you get there (hope all those grots help you get there) you will kick butt.
A -very- basic force you can fight with Right Now:
1 Warboss
Unit of 30 Grots (with slaver)
Unit of 30 Grots (with slaver)
Unit of 19 Grots (with slaver)
Unit of 16 Orks and 1 Ork Nob to lead them
Unit of 17 Orks and 1 Ork Nob to lead them
So out of your 79 Grots, paint each unit in different colors.
Out of your 39 Boyz, paint 3 of these to match the Grots (these are your slavers)
Also paint 1 of these guys to be your Warboss
Finally, paint up 2 units of Orks... use 2 Boyz as Nobs and paint them to match your Boyz Units, but also make them stand out somehow so everyone knows that they are Nobs.
I suggest buying the Codex: Ork first. Play using the above suggestions and learn how they work. After you have done this for a while, you will be able to better decide what to get next!