When you're working as a freelancer you can't simultaneously get unemployment benefits. More precisely, you can do part-time work (less than 15 hours per week), but it will reduce your benefits if you make more than EUR 165 per month (profit, not revenue). If you temporarily want to do more freelance work, you can de-register as unemployed for those phases. Talk with your caseworker.
If you want to start a business (i.e. want to become permanently full-time self-employed) you may be able to get the Gründungszuschuss (ALG I + EUR 300 for 6 months). However, this requires a convincing business plan.
If you take on any work (also part time, also freelance), you must notify your case worker in advance. You also have to clearly document your earnings.
Aside from the ALG concerns, working as a freelancer in Germany involves a bit more bureaucracy. You have to determine whether your activities are freiberuflich or gewerblich, which can result in different taxes and formalities. Some businesses cannot be taken up freely but require permission first. In any case, you'll have to register with your Finanzamt, and will be required to file taxes in the future. You would likely do business as a sole proprietor (Einzelunternehmer). If this is just part-time work, you can likely opt into the Kleinunternehmer-Regelung which means you don't have to handle VAT/USt. While this is convenient for B2C services, you do need a VAT ID for B2B services when your client is in another EU country. Both Freiberufler and Kleinunternehmer benefit from simplified accounting requirements.
