If you're interested in flipping domains, here's one way to start:
1. Setup a paypal account
2. Setup a GD account and add auction package. (like 5.99 per year)
3. Get familiar with GD domain auction platform
4. Research what is selling by watching the most active domains on GD auctions, and reading domain forums and blogs. (most important)
5. Research and buy similar(this part can be complicated) domains in this forum at wholesale or even lower prices(don't rush...be patient).
6. Put them on 7 day auctions on GD. Domains on GD auctions can be from any registrar.
7. Advertise your GD auctions on this and other sites
8. Repeat 3-7
If you did your homework and bought the right domains, you should see decent profits.
Of course, your budget will determine how much and what you can buy. And notice that I didn't mention any amounts you'll need spend to buy your similar domains (3). You have to stick to domains with room for you to profit, and that are actually selling.
I'm sure there are lots of other ways to flip domains but that's just one.