You can take possession as soon as you get your tax certificate, but you can't just throw someone's stuff out into the street. If the property is vacant, then take possession. If it is occupied, you must give written notice to vacate and then must wait 6 months before filing an ejectment lawsuit.
It is critically important to take possession as quickly as possible, because this is the event that starts the judicial redemption clock ticking down. There are several redemption time periods in Alabama. It is tricky to figure out yourself, but fairly easy to explain. Contact me privately for more information.
If you have a tax deed (available 3 years after the tax sale) then you do not have to give the written notice and wait 6 months. You can file for ejectment immediately.
Often, once the investor indicates that it plans to take possession of the property, the owner will begin redemption. Be aware that even if you have a tax deed, the owner can still redeem.
If you file an ejectment lawsuit and the owner counterclaims for redemption, your legal fees will be added to the redemption cost the owner will have to pay.
If the owner files a lawsuit against you to force redemption, and then you counterclaim for ejectment, you are NOT entitled to recover your legal fees. The owner does not get legal fees, either, though. Why would you counterclaim for ejectment? Your counterclaim would say, basically, "We don't think the owner will be able to redeem. Judge, we want you to decide the redemption price tag, and then give the owner a short period of time to cough up the money and, if he doesn't, we want you to order him off the property so we can go ahead and take possession."
Because of the legal fee rules, it is tactically important that you be the first one to file a lawsuit.
It is a good idea to get liability insurance whether you have a tax deed or tax certificate. Premium will be around $50 per year. Not all insurance companies will write casualty insurance on a tax certificate. Since you have possession rights but not ownership, your insurable interest is limited.
If you do purchase casualty insurance, and the property contains a residential structure, then if the owner redeems, they will have to reimbursed you for the premiums, plus 12% interest. Remember, the property MUST contain a residential structure in order for you to be compensated for the premiums.
Good luck!