How soon should I trademark my domain name ?

You have to show some real-world use of the mark, so the sooner you can demonstrate it in a few places the better. Talk to a trademark attorney about this.

You should talk to an IP-focused attorney fairly early in the business anyhow. If IP will be important to your business down the road, it's a good idea to get someone in your corner early on. You won't need to spend much money early on, and each modest investment in doing things right will save you a ton down the road (in money, and in dealing with b.s.).

Trademark is a fairly simple procedure, and in six months or so you'll have a registered mark. But you can mess this process up slightly and cause yourself some re-work and delays. Go on Yelp and read reviews of different attorneys that are specifically classified as "IP & Internet Law". You'll make an important friend.

Meanwhile, no... the domain name is not a trademark in itself. If you mean, is "ebay.com" a trademark in itself. The answer would be, "it depends." Ebay Inc., may have chosen to trademark both "Ebay" the word mark, "Ebay.com", another word mark, and their "ebay" logo. For your purposes, it's likely that simply trademarking "yourcompanyname" as a word mark, properly categorized, would suffice for now (without the ".com"). But much depends on your proposed company and/or product name, what else is out there both on the USPTO site and on the web, and other factors.

Again, I feel that your questions are rudimentary enough that you're just learning Trademark 1A, and you could benefit from spending a couple hundred bucks on a really good independent trademark attorney.