Your ability to get your record sealed will depend on a number of factors, and it would be best to contact attorneys who handle juvenile cases and get an idea of how much they would charge for this matter. If you want to do further research yourself, the law for sealing of juvenile records in is the Texas Family Code Section 58.003 and is available online.
You can seal the record on your own. It requires filing a petition. Many courts will have a law library where you can speak to someone and they will be able to help you with a petition. With that being said, you may also be able to simply search on Westlaw or lexis at the law library for a sample petition.
Ultimately, you want to speak with an attorney. I know you do not think you can afford an attorney but many attorneys will work with you on their fee. Many small firms will offer a payment plan that will work with your available funds. Try talking with a younger attorney in your area that is interested in getting the experience of sealing a record. We often offer payment plans to clients to make sure that they get the representation that they deserve. But note, if you find an attorney who is willing to work with you on payment to follow through with your end of the deal. Many older attorneys that I've spoken to have refused to offer payment plans anymore because they were burned too many times in the past.
This answer is simply to provide information in response to your question. It does not create any attorney-client relationship or an obligation on my part.