The invoice template CM2DD linked to is from the following book:
http://www.amazon.de/Gr%C3%BCnes-Licht-Existe...It probably can't hurt to take a look in there.
I'd suggest that you do not opt for the Kleinunternehmerregelung, by the way. The only "advantage" it offers you - not having to charge Umsatzsteuer (also called Mehrwertsteuer) - is actually a disadvantage in my opinion. If you don't charge it, you're saved some paperwork. If you do charge it, you can offset any Umsatzsteuer you *pay* for your business.
Let's say you buy a new computer and pay EUR 150 Umsatzsteuer on it. In the same tax period you write several invoices and charge a total of EUR 120 Umsatzsteuer.
EUR 150 - EUR 120 = EUR 30. Which you get back from the Finanzamt.
Hopefully, of course, the Umsatzsteuer you charge will soon exceed the Umsatzsteuer you pay because you have so much work. But even then it's nice to be able to offset the Umsatzsteuer you pay.
And should you ever earn more than the 17.500 or whatever it is, you don't have to go through the process of revoking Kleinunternehmerreglung, which could even involve paying back tax, if I'm not mistaken.
If you do opt to pay Umsatzsteuer right from the start, make sure to get both a "Dauerfristverlängerung" and "Besteuerung nach vereinnahmten Entgelten". The first means that you have more time to report your Umsatzsteuer and thus to pay it. If you have to do it monthly, you'll have until the 10th of July to report your Umsatzsteuer for May. Without a "Dauerfristverlängerung" you only have until the 10th of June. The "Besteuerung nach vereinnahmten Entgelten" is also quite handy. If you don't have it, you have to pay the Umsatzsteuer you charge on your invoices to the Finanzamt as soon as you have sent the invoice. If you do have it, you don't have to pay the Umsatzsteuer to the Finanzamt until your customer has actually paid you the money.
As others have said, however, you should really talk to a Steuerberater. There's so many things you can deduct, like phone bills, rent, electricity, insurances ...
Good luck!