There are several possibilities - however, rather than merely guess - wasting both your time and money, it is best to begin the diagnosis at the battery and work towards the ignition switch in the steering column.
To start with, you will need several tools and the vehicle repair manual, which contains the wiring schematic / diagram. Tools required (at a minimum): basic screwdriver set, volt-ohm meter, jumper wires at least 3 feet length, small 1/4" drive rachet and socket set.
The first steps simply involve checking and confirming the battery, the batterty terminal conections, and related wiring condition. Locate the battery under the hood, check (visual inspection) of the battery itself along with the clamps, cable connections at both ends of the positive and negative (ground) cables. These should be clean and tight, with no looseness or corrosion. Insure the battery, terminals / battery clamps are in good, operational condition.
Next steps are each related to the ignition switch / lock cylinder located inside the steering column. Using the repair manual, remove the underside and top covers above and below the key switch.
Next - with the small gauge jumper wires and the electrical schematic - locate the 12V supply wires by color - which feed into the key-activated ignition switch - there should be only two. With the key insterted into the ignition switch, and turned to the 'ON' position - place (secure) the two small wires from the incoming 12VDC source - such as the battery terminals (positive and negative) posts. Then in turn, connect and just touch off the corresponding switch wire terminal connections, as you are bypassing (jumpering around) the ignition switch and keys - to isolate the issue.
If the engine and starter operates, like the car will try and start - then pefrom the next step; a 'wiggle test' for the incoming 12VDC wires to the ignition switch; while gently manipulating the wires - to see if the momentary contact will allow the switch to work, and the engine starter motor to engage.
The goal is to test and to confirm whether the cause of the 'no start' issue is in the switch itself or in the related wiring.
You should be able to replace either the ignition switch from around $25.00 and the ignition lock cylinder from around $149.00
Igntion starter switch (with auto transmission) = WVE/AIRTEX/WELLS part #1S6491 - or it's equivalent
Ignition lock cylinder (with keys) = BECK/ARNLEY part #2012437 - or it's equivalent