Do I need a lawyer to apply for citizenship?
No lawyer necessary. You are either a legal permanent resident & fully qualify for citizenship, or you don't. You are either capable of filling out a form accurately, or you aren't. Everything is designed to "do it yourself" - as long as you can read & follow instructions. People with dodgy claims or complications hire attorneys to help them, but if you don't qualify, you can spend all the money an attorney wants without getting any approval.
You either qualify or you don't and if you want to spend money on legal advice and for them to fill in the paperwork that is up to you, it will change nothing at all, you either qualify or not a lawyer makes no difference
If you are asking about US citizenship then you either qualify or you don't. Many people get assistance with completing the various application forms so they don't make a mistake that will delay the processing of their application. The assistance does not have to be an attorney, and you should search "immigration rights groups" for wherever you live to ask about sources of low cost assistance.
Most people don't need lawyers to apply for citizenship. As long as you trust yourself to file the paperwork correctly you should be fine.
You'll need a lawyer at the USCIS office once you have been arrested because the comprehensive FBI background check revealed that you are a fugitive of the law, ordered to be deported.