91 Subaru Legacy Wagon and it will not start. Any suggests on what I should replace first.

The lights come on when the key is turned but just a single click that's it. It has started a couple of times just fine then nothing again. Battery cables are clean and tight. Voltmeter shows that it is good. Hooked up a battery charger just in case but nothing. Jumped the car and it started right up. Then the next time it wouldn't even start with a jump. Since it is a manual I can bump start it and that seems to be the only way it will start now. I need to get back and forth to work and I can't depend on my husband pushing me or asking someone else to. Any suggests on what I should replace first. I've been told it could be the starter, starter solenoid, ignition, or the alternator. I don't have very much money and need to get it fixed asap. Thank you for your time and any input is very much appreciated.