Typically, your machine will tell you how to enter BIOS setup when it boots. If not, the owners manual or vendor's support page for your machine should provide the info. The method likely will be a key press, a simultaneous combination of keypresses, or a series of keypresses, usually, but not always, right at boot; unfortunately, there really is no standard method, and even among machines from a single vendor there may be different ways of gettin' there - sometimes even multiple ways.
Here are some common ones, though there are lotsa others:
Acer: (Depending on model) F1, F2, or CTRL+ALT+ESC
AST: (Depending on model) CTRL+ALT+ESC or CTRL+ALT+DEL
Compaq: (Most) F10
CompUSA: DEL
Cybermax: ESC
Dell: (Depending on model) F1, F2, F3, DEL, or (after boot) Fn+F1
eMachine: DEL
Gateway: (Depending on model) F1 or F2
HP: (Depending on model) F1 or F2
IBM: (Depending on model) F1, F2, or (older models) CTRL+ALT+INS after CTRL+ALT+DEL
IBM Thinkpad: (current models) After boot, Windows Key or Start Key >Programs>Thinkpad CFG
Micron: (Depending on model) F1, F2, or DEL
Packard Bell: (Depending on model) F1, F2, DEL
Sony VAIO: : (Depending on model) F2 or F3
Tiger: DEL
Toshiba: : (Depending on model) ESC or F1
WinBook: : (Depending on model) ESC and/or F2
Once you're into BIOS setup, use your arrow keys, function keys, or your tab key (as directed) to navigate to where you wanna change settin's, follow the onscreen directions to effect and save your desired changes, then follow the onscreen instruction to exit BIOS Setup and continue with booting (Most often ESC). Its a good idea, when done with whatever called for you to modify BIOS settin's, to re-enter BIOS Setup and restore the default settin's, which usually may be done simply by selectin' the "Default" option then the "Save Settings" option and exiting.
Enjoy