This is a very wide subject... You can search online for CRM/ERP/Fiscal Management softwares for Creative Agencies/Studios. You'll find hundreds of solutions, guides and software comparisons.
By the way if you are a single freelance you'll hardly need any of these. Sometimes a well organised spreadsheet will do the work of keeping track of the ongoing works, tasks, deadlines, etc.
Personally I use some online spreadsheets (with daily backups) and a small fiscal management software for invoices and similar documents.
In the past I tested a few softwares but I always found them an added layer of complexity in my daily work after a while, I just wasted time (and money) over them and they did not solve me any problem or improved my productivity, in my case.
Unless there are very good reasons to use these kind of softwares (large number of clients, big agency, sales departments, etc) I usually discourage to use them for a single freelance.
Also I always adhered to the "KISS principle", which stands for slightly different and very good advices like these:
- Keep it simple and stupid.
- Keep it short and simple.
- Keep it simple and straightforward.
... keeping that in my mind always avoided me many complications.