If I was convicted of a misdemeanor petit-theft 7 years ago, can I get it removed from my record myself?

If you are eligible, you can potentially file the expunction request yourself, but if you paperwork isn't in order, then the judge could deny your request and you will have wasted your filing fee.  The caveat is "if you are eligible."  Depending on how your case was actually pled, you may or may not be eligible.  Even if you don't hire an attorney to do the expunction, have an attorney review your court papers to see if you even qualify for an expunction. For any attorney to know if you are eligible, they have to see the actual documents.   You don't want to waste funds or effort in filing an expuction if you are not otherwise elible for the expuction.
Step one is basically determining eligibility.  Step two is finding a way to get it done.  Attorneys can be pricy, but you do have some options.  The first is to shop around.  Some attorneys really specialize in expunctions and can do them quicker and cheaper than other attorneys.  So, look for attorneys who have specific experience in expunctions.  Don't get the attorney that lists twenty something areas of practice... find the one that knows this area and can get it done quickly and cheaply for you.  You can also visit with the attorney to see if the jurisdiction where you file is willing to waive filing fees for persons with limited incomes.  Not all jurisdictions are generous about waiving filing fees-- but some will and it can save you a couple of hundred dollars.  The second option for you is to hire an attorney for the limited purpose of drafting your documents for you.  You would then be responsible for filing and presenting the documents to the court.  The third option is to tap into an online form bank and try to work your way through the process.  The process begins with filing the application or petition for expunction.  You are then required to notify the prosecuting offense and any agency that is reporting your data.  Once every agency is properly served with notice of the expunction, then you can set the expunction for a hearing to present your evidence regarding how and why you are eligible for your expunction.  After the judge signs your order, you will need to get enough copies to send to each of the respective agencies.  This is their instruction to delete the information from your records. 
If you are not eligible for an expuction, you are not necessarily stuck.  You may still have a couple of different options for relief... they tend to be more challenging and expensive... but they are still options.  One is to see if you are eligible for a non-disclosure program.  This is where law enforcement gets to keep your conviction info, but they pull it from the general view of the public.  This will help if you are looking to shield the conviction from employers.  Another option is to challenge the underlying conviction.  It is extremely hard to overturn plea bargained convictions... but it's not unheard of.  It does require a consultation with an appellate attorney, not an expunction attorney.