This will depend on exactly which model of laptop and TV. Look at the TV and determine what ports it has. Then look at the laptop for the same information. You may need to enable the external video ports in the BIOS. Turn off the computer and press F2 repeatedly after turning the unit on. (Some computers use Esc, F1 or F10. There will be a quick splash screen with the information.)
Then it still depends. If your TV and your laptop both have a VGA port, get a VGA M/M cable. (Often this is blue - it's a D-sub 15 pin connector.) This is the simplest option. Turn off the TV and the laptop and connect the cable. Then turn on the TV and set it to the VGA source (look for a button marked Input or Source on the TV or remote). Then turn on the computer; with most modern OS versions, it should set the correct resolution. You will need to enable the external monitor. This will depend on the OS. In Windows 7, right click on the display (where there are no running programs or icons) and select Graphics Properties. Then set the monitors to use the external monitor. Click Apply and then confirm the setting. Note: you'll need to also connect an audio patch cable (3.5 mm) between the laptop's headphone jack and the TV's VGA audio input if you want to use the TV speakers.
If your laptop and TV both have an HDMI port, you may be able to use that cable. You'll need to check the details on the TV. Not all TVs will read the computer's signal on the HDMI port. (The data isn't the same as from a set-top box like a Blue-ray player or cable box.)
If the laptop has a DVI port, depending on the TV, try either a VGA to DVI adapter or a DVI to HDMI adapter.
A few older laptops may have have an TV out (looks like S-video) port. In that case, you can connect to a similar port on the TV with a dongle and the cables.
For a TV with only composite or component input and a laptop without a TV out option, you'll need a VGA to composite or a VGA to component converter. These convert the RGB signal from the laptop to one that the TV will understand (component is YPbPr). See http://www.ramelectronics.net/audio-video/video-converters/vga-rgb-hv-to-composite-video/c10000-c11200-c11213.html?sorter=productname-en-asc
Please add more information about your laptop and TV for additional help.