The most basic lock for your front door is a password. Ensure that every computer on your network requires a password before anyone from the network can read your information or write to your hard drive. If a password isn't required, there is no front door at all. If you're not sure how to ensure that passwords are necessary, I strongly recommend getting hold of a computer expert, or at least a very good manual.
Most computer systems will not password-lock someone sitting at the computer itself. There are ways to do it, but there's usually a way that someone at the computer itself (not on the network) can get in and change the passwords. This is to prevent the computer from becoming an expensive doorstop if the passwords are forgotten. This does, however, mean that you still need physical security.
Changing forgotten passwords isn't easy, however. It's better not to forget them in the first place. If your system has a 'master password' that has access to everything, make sure two people in your company or household know that password. If there's only one, what happens when that person is on vacation on that tropical island with no phones?
Passwords are only as secure as they are difficult to guess - if your password is your name, for instance, or the word 'password', it's like putting a lock on the front door and never bothering to actually lock it.
There are a lot of suggestions for how to make passwords difficult to guess - here're a few of them:
no less than eight characters long
include both upper and lower case letters, numbers and punctuation marks
don't use anything which can be guessed by someone who knows you or has your information - no names of family members or pets, no licence numbers or passport numbers or phone numbers or similar, not a street address (current or past!), not any words which are visible from your desk (like the brand of monitor)
no legitimate words in any language, brand names or logos
no swear words
not a simple substitution (ABC as 123, to as 2, Ziggy as 2166Y)
not the same password as on as another computer, or the same one you had last year. ANY password can be figured out in time, and if someone guesses one of your passwords they might try the same thing for another computer
not a common misspelling of a word
Suggestions for good passwords include
take something you'll recognise - a line from a book or a line of poetry - and use the third letter of each word. Include punctuation (but not spaces)
a really, REALLY bad misspelling of a word
two words from different languages stuck together with punctuation marks
a short phrase
Think up other suggestions. For passwords, weird and idiosyncratic is good.