There are two ways. The quick and easy way is the use of passwords.
The better, but more complex way is to use user level security
I suggest you start by reading
http://support.microsoft.com/default.aspx?scid=kb;[LN];207793
Access security is a great feature, but it is, by nature a complex product
with a very steep learning curve. Properly used it offers very safe
versatile protection and control. However a simple mistake can lock
everyone including God out.
Practice on some copies to make sure you know what you are doing.
If you have not already done so and if this database is shared over a
LAN, I suggest splitting the database so only the data that is changed is
located on the LAN sever and each user has their own copy or a "Front End"
that links to the data on the Server. This will work faster and offer more
reliability.
