As I myself have been preparing to become a real estate investor, I have collected quite a bit of knowledge over the years and have created somewhat of a checklist or a to do list before you take your first plunge.

1. First and foremost, join an investors group.  Look locally for a group that meets regularly in person, if you cannot find one then join some online.  They will prove to be a valuable asset as you progress thru your real estate business.
2. If you do not already have a real estate attorney, go find one and be certain that the attorney is a quality attorney.  A quality attorney can save you a bundle in protecting your assets and bad deals.  If you do not know how to find one, ask your investors group from step 1.
3. Ask the attorney for a referral to a CPA who works with investors.  Do not go for any CPA, you want one who has a great deal of experience in handling taxes for people with investment property.
4. Research the local and state laws that affect your real estate operations and make a reference guide to keep close at hand.  One example would be knowing the process for eviction in your town, because if you make a mistake, not only have you not received any rent, but your request for eviction could be thrown out and you could miss even more rent correcting your mistakes.
5. Open a PO Box and have all rent payments be sent to it.  Never disclose your home address to tenants.
6. Decide what your investment of choice will be, as each investment offers different pros and cons and you will want to match those pros and cons with your goals as an investor.  For example, commercial properties are great because as an investor you can use a triple net lease and the tenant would be respobsible of building maintenance, utilities, insurance and taxes.  But a large con is in the event you need cash quickly, your market for a buyer is other investors which means they are looking for a bargain and you will not be able to sell the property at full value.  Not to mention commercial property can lay vacant for months or years at a time.
7. Make certain you take proper caution when screening tenants.  The worst thing you can do is take the first tenant that calls you and fill a vacancy.  Just as a sick child goes to school, the rest of the class gets sick within days.  A bad tenant can disrupt your great tenants and even while you are attempting to evict the "bad apple", the eviction process could take 60-90 days and it that time you could loose 2 or 3 quality tenants, not to mention possible property damage. Filling vacancies without due diligience is often much more costly than taking the extra time to properly screen tenants.
8. Make certain that when you speculate potential cash flow and ROI of an investment, you factor in up to 2 months worth of no rent where it applies.  If you can still operate those two months without going in the red, you have a much better chance of weathering times when you have multiple vacancies at the same time.