In order to get started you'll need Odin and Heimdall. You won't need Odin most of the time -- Heimdall is more flexible -- but Odin's user interface is vital for a rare but nasty case, when you can turn on your phone but the screen and buttons stay black. In this case, Odin detects the state of your phone, and makes it possible to boot into download mode with a lot less guesswork.
When using Heimdall, first connect your phone in download mode (turn off the phone, then turn it on while holding down the volume down button), then use Heimdall's zadig utility to install the drivers Heimdall needs. Select Options/List All Devices, then choose your phone, then hit "Install". Heimdall is now good to go. When using Heimdall, for the most part just copy the command lines other people give you. If you want to actually understand Heimdall, don't look at me.
If you completely brick your Galaxy Tab (the older P1000 model; I have no experience with the newer ones) then you have two options: install a stock ROM, or a custom one. It was much easier for me to find and download the custom ROMs, especially for the P1000N. Be very careful; if you have a P1000N and you install stock P1000 firmware, you'll get the problem described above, where the phone will kind of turn on, but you'll get no screen. The P1000's hardware is a little bit different in just the wrong place, it seems.
If you want to install a custom ROM, Cyanogenmod is a fairly stable and safe way to go, and it will bring your phone into the Gingerbread era. Cyanogenmod 7 is easy to flash and there's a Heimdall package for it on XDA developers. You'll need Google apps for Gingerbread, too (known as gapps and available at goo.im -- gapps-gb-20110828-signed.zip is the latest for Gingerbread).
Follow the instructions on the XDA developers page, taking note of the kernel and bootloader to use for P1000L and P1000N, and you'll have a Gingerbread tablet with very little hassle. You will need to flash a new modem if you use this ROM with a P1000N, though, and that might break your GPRS/data. It broke mine. The JP3 modem is the one you most likely want for a P1000N.
If you want Ice Cream Sandwich, it's a bit trickier, especially if you have the P1000N. The easiest way is to get HumberOS's builds, since he makes builds compatible with all the P1000x models. His blog is in Portuguese but the posts about Cyanogenmod are in English. Find the latest one of those and grab HumberOS's build. This will be flashed using the recovery mode on the phone; put the zip on the phone's internal SD card, reboot into recovery, and let the installer do the rest. You need to follow HumberOS's instructions carefully, though, especially about flashing the CwM recovery ROM first (the stock ROM and the ROM installed with Cyanogenmod 7 above are both insufficient for installing Ice Cream Sandwich).
Long story short: if you really want custom ROMs on your P1000N, don't. It's not worth it. But since you won't listen to me (I didn't, after all), I've given you most of the links you need to follow.
And backup your data first; I lost all the apps I'd downloaded and all my contacts (the Heimdall flash will do that, it's like getting a brand new phone).