Here's how I started...
1) If you think a property is a good deal (based on some rough estimation of rehab costs and ARV), get it under contract with a contingency for a 7 or 10 day due diligence period (this let's you back out if you later realize it's not a good deal).
2) Once you have the property under contract, get an inspector to come out an give you a report on the condition of the property. He should be able to tell you about the condition of all the major structural and "expensive" components, such as foundation, electrical, plumbing, roof, hvac, etc. A good one will also tell you about the obvious stuff as well, in case you really have no experience. A good inspection will run you between $150-400, depending on the inspector.
3) Once you have your inspection report, find a contractor (or more than one) to come out and give you either an estimate or bid. If you plan to have the work contracted out, you can ask the contractors for a bid, and they will provide you a full statement of work with their price proposal for free (they expect they might get a job out of it). If you don't plan to hire contractors, then find a contractor who will -- for $100 or less -- come out and give you a "pre-purchase estimate." This will be a ballpark estimate of how much it would cost to rehab the property (most contractors will do this for a small fee). Keep in mind that the numbers may be higher or lower than your actual costs (depending on how you do rehab), but it's a good ballpark.
4) Have your agent (or an agent or an appraiser) do a BPO or comp analysis for you to determine your ARV. If you're not working with an agent, you may have to pay a couple bucks for this, but it's better than trying to guess an ARV with no experience.
Now, for about $300-500, you have all the information you need -- you know the purchase price (you have the property under contract), you know what's wrong with the property, you know how much it will cost to fix, and you know what you'll likely be able to sell for after rehab.
Sure, it's cutting into your profit, but would you rather risk $500 upfront, or the $100K you're paying for the house?