How do I make an invoice for software development services?

I recently did a job for a company as a side business. Now that the task is completed I am not sure if I have to include tax or not at the bottom of the invoice.

sounds like you don't. if you charge tax for your service, you have to be registered first. and obtain a tax account number. it appears that you have not registered. so you don't charge tax.

http://office.microsoft.com/search/redir...
Then Invoices; Services invoice with tax calculation.

You need to check with your state, etc. To see if you will be required to charge tax for your services. You can do this by contacting your state revenue office. If you need to charge tax you want to get information from the state on where to send the tax because if you charge and collect tax and do not send it to the government you will be liable for the amount as well as penalities, fees and interest.

If you do need to charge tax the revenue office can instruct you on using your social security number as the tax id number and the proper forms. Not all independant consultants need a seperate tax ID number. If you are the sole proprieter and not incorporated or an LLC than you can use your social as your tax id number instead of applying for a seperate number. The IRS site as a wealth of information in their help area on who needs a tax ID number.

Now you can't tax them for what YOU will pay in income taxes on your services. The only tax you can charge them is if your services qualify for a tax charge in your state and if so that money must be sent in to your state tax revenue departement. Who will be happy to help you with any information you need.
You will still be required to pay income taxes on the money. Depending on the amount of money they paid you if it's more than $600 then they should supply you with a 1099 at the begining of next year they will also send a copy to the IRS which means you have to pay the appropriate income taxes on the money. www.irs.gov has a large amount of information on the subject.

Good luck and I hope that you will be able to do a lot more consulting as a side business. I have been doing consulting solely now for quite awhile it's wonderful to be able to have more control over your work!

Since you did work as independent contractor, you can just do an invoice without the tax.