How can I encrypt a file?

You can use openssl to encrypt and decrypt using key based symmetric ciphers. For example:

openssl enc -in foo.bar \
    -aes-256-cbc \
    -pass stdin > foo.bar.enc

This encrypts foo.bar to foo.bar.enc (you can use the -out switch to specify the output file, instead of redirecting stdout as above) using a 256 bit AES cipher in CBC mode. There are various other ciphers available (see man enc). The command will then wait for you to enter a password and use that to generate an appropriate key. You can see the key with -p or use your own in place of a password with -K (actually it is slightly more complicated than that since an initialization vector or source is needed, see man enc again). If you use a password, you can use the same password to decrypt, you do not need to look at or keep the generated key.

To decrypt this:

openssl enc -in foo.bar.enc \
    -d -aes-256-cbc \
    -pass stdin > foo.bar

Notice the -d. See also man openssl.

For one-off cases you can encrypt using zip and a password. While not as strong as key based techniques (because it is hard to have a good password) it is probably fine ad-hoc situations.

Command line looks like this:

zip -r -0 -e encrypted_file.zip /path/to/files

-r to recurse directories.
-e to encrypt