1994 Hyundai Elantra OBD or what?

Most OBDII readers are OBDI compliant just check before buying

RETRIEVING CODES (NON-SCAN)
Before entering on-board diagnostics, refer to SERVICE PRECAUTIONS. Turn ignition switch to OFF position. Locate Data Link Connector (DLC), next to fuse block. Connect voltmeter positive lead to Data Link Connector (DLC) MFI diagnosis terminal and negative lead to ground terminal. See Fig. 1 .
Turn ignition switch to ON position. Voltmeter (analog) will display trouble codes by 12-volt pulses of voltmeter needle. Trouble codes are retrieved in numerical order. A constant repetition of short 12-volt pulses indicates system is normal. If system is abnormal, voltmeter will pulse between zero and 12 volts.
Codes will appear on voltmeter as long and short 12-volt pulses. Long pulses represent tens; short pulses represent ones. For example: 4 long pulses and 3 short pulses indicate Code 43. After recording fault code(s), perform necessary repair(s) to indicated circuit(s). See FAULT CODES.
Fig. 1: Identifying Self-Diagnostic Connector Location
Courtesy of HYUNDAI MOTOR CO.
RETRIEVING CODES (SCAN TESTER)
NOTE:If scan tester is needed to perform diagnostic test with key in START position, an auxiliary power source is needed to operate scan tester. Use optional Battery Harness (09391-33500 ).

NOTE:For specific operating procedures using scan tester, see instruction manual.

Before entering on-board diagnostics, refer to SERVICE PRECAUTIONS. Turn ignition switch to OFF position. Locate Data Link Connector (DLC), next to fuse block. Connect power source terminal of scan tester to cigarette lighter socket. See Fig. 2 .
Connect scan tester to Data Link Connector (DLC). Turn ignition on. Read and record scan tester self-diagnostic output. Perform necessary repair(s). See FAULT CODES.

The easiest way for you to get started with OBDII is with your own vehicle. OBDI was the start and it's more advanced year by year. Buy yourself a reader. Once you have got that, read your car. The next part is finding what the code it gives you means. You can get manuals on this. Most manufacturers use the same codes for powertrain (engine related codes) but they do not use the same for other things like Airbag. Body controls etc. Etc. They are called manufacturer specific codes. If you learn how to diagnose the problem for powertrain related codes you are halfway there. You have to research the codes. Get the codes off your cars PCM powertrain control module. Then try to find out what the codes mean by manuals, googling etc. Even by asking us. Thats what we are here for. Just remeber one thing. A code may tell you there is a fault on a certain sensor. That doesn't necassarily mean the sensor is the fault. You have to look at all related parts. Ie. If a code say's misfire on#1 cylinder you have to look at all the parts that are related to #1 cylinder plugs. Leads, coils, CKP etc. Etc, . Hope this helped. If you need more help let me know.