If we are talking about flipping houses, then I would do all cash deals ideally. If you don't have the cash, then you need some kind of line of credit or possibly a partner who's only duty will be $$$ but that requires a lot of trust (and experience on your part) otherwise look to other sources of non interest loans say from family.

If we are talking rental property, that means you are going to want to be cash flow positive. For that to happen, you need to put 35% down in order to charge competitive rents and still be cash flow positive. I always run the numbers on the conservative side. For example, if a single family house, then I always figure 1 month per year unrented. Or if we are talking about a 6 unit apartment building, I figure one unit is always unrented or undergoing remodeling etc..

The absolute best resource out there on the internet in my opinion is biggerpockets.com

On a side note, the one thing I have learned about flipping houses is this- it's best to flip houses in expensive neighborhoods, better margins! For example- whether we are talking a small house in a crappy neighborhood or a large house in an expensive neighborhood, a mid to premium kitchen covering say 700 square feet costs the same in both houses. But it drives far more value in the larger house for a variety of reasons. I could go on and on but this isn't the place.