Hi Wmanidi
Putting together all that I and the others have said: buy any quality EHD (75 USD as stated above should easily cover it). Make sure the EHD has at least the storage capacity of your Int HD, preferably a little more.
A Firewire 800 connection allows significantly faster transfers than USB, but USB will do the job if that's all you can get or afford.
Next; connect the EHD to your Mac. At this point, if the EHD is not factory formatted for the Mac, you will need to go into Disk Utility (found in Applications > Utilities > Disk Utility). You should see the mounted EHD listed on the left. It will show as a Name (say LaCie 1GB) with a sub heading. Click on the Sub Heading. Now look to your right and near the top it will show First Aid-Erase-RAID-Restore.
Click on Erase. Under Format, choose "Mac OS Extended (Journaled) and under Name, choose one for yourself, say, TM Back Up. Then Click on Erase and your EHD will be formatted, ready for use.
Now, go into Preferences > Time Machine > Select Disk. Choose the TM Back Up (or whatever you named it) and start the back up. (NB make sure the slider on the left is in the ON position).
As stated, the first back up will be a lengthy process because you are backing everything from scratch (even longer if you use USB rather than Firewire 800).
When it finishes, it will automatically (repeat automatically) back up Hourly for 24 hours; Daily for a month; thereafter, Weekly. This takes hardly any time at all.
What this means is that at any point in the future, you will have hourly back up for the previous 24 hours, then daily back ups for a month and so on.
I think that covers everything.
Cheers, Ian