I want a database tool where i'm able to easily add information into it.
I've 2 tables, clients and bills. The database tool must show me input fields from the customer and bills. invoice includes a foreign answer to customer. The input fields of bills should have a dropdown where i'm able to choose the client and so i will not need to copy paste the ID from the customer.
The tags in your question suggest you might be creating test data. Have you thought about something like RedGate's Data Generator?
If you wish to go into the sample data by hand (that's the way i understood your question), Microsoft Access seems like the tool of preference here:
Produce a new Access DB and connect to the SQL Server tables. Go into the clients directly in Table view. For that bills, make use of the wizard to produce a form, then produce a combobox that references towards the clients table.