Let us assume that you are starting out and that you do not have a massive bankroll. As such, as folks suggest, Cards Chat Freerolls are a great place to start. But as some have noted, you need to play for quite a long time just to win a couple bucks. And there are only a couple per day per site.
So...what I would recommend, is that you start out simply with the 50 cent Sit N Go. (NOT SPIN N GO). These games are relatively soft. About 50% of any given table is made up of regulars, whilst the other half are entertainment players. So you only need to beat a couple regs to get into the money...and you can play nitty, TAG or even LAG and still get ITM.
The bigger bonus though is that you can play several of these games at the same time. If you've got yourself a HUD like PokerTracker4, you can more easily track what is going on in these games despite playing 4-8 (or more) tables at one time.
The other benefit is that these games are short. 45 mins and most single table SNG's are finished.
So that's my advice!
Cheers,
JT