Flipping a house (buying it for the purpose of reselling it within a short period of time to make a profit) means taking a chance that you'll find a buyer and you won't be stuck with the house month after month. People who buy a house, renovate it, and then sell it are sinking more money into the house, but they've got a more desirable product. This is called flipping a fixer-upper. Most experts in the home remodeling field advise that you're better off turning a cheap house into a nice house than taking an already nice house and upgrading it into a really fancy one, as it's unlikely that your potential buyers have the same taste as you have and you're probably not going to get back as much as you put in.
If you're just starting out in the fixer-upper flipping business, buy and sell only one property at a time so you don't go into bankruptcy. Each house needs its own repairs and attention, and you can't spread yourself too thin. Pad your figures regarding how much time and money the renovations are going to take to avoid unpleasant surprises. How much of the work are you going to do yourself and how much do you need to call in other people for, such as electricians or plumbers? You'll have to coordinate who comes when and supervise the workers, unless you want to hire a general contractor, who will just turn out to be another expense. Don't try to skimp on the quality of the job, or you may have to end up redoing it.
After you finish your renovations, don't ask for an unrealistic price or you'll find that the house just stays on the market.