2001 Ford Focus ZTS wont start

Yesterday as I was trying to leave from work I went to start my car and it turn over. The radio worked and the dome light in the car seemed fine. I would get an intermittent clicking sound from behind the dash but it was not constant. I had the driver side door open and the warning ding would go dddddddddddddddddddddddddding when I turned the key and the seatbelt light would flash. The battery seemed to be OK but there was a bunch of corrosion on the +ve terminal. I cleaned it off (not well) but that didn’t seem to help. I tried to jump it but it just did the same thing. I was first thinking the solenoid but information is probably out of date becuase I’m not even sure if they still use those anymore. My reading online has suggested everything from the starter, the starter relay, the solenoid on the starter, somthing about cable (going to starter). I would like to try to fix it myself becuase I fear a mechanic will just replace everything until it starts to work and charge an arm and a leg for labor on an inexpensive part. Does anyone know and diagnostic tips to help me isolate the problem? My car knowledge is very limited but I am willing google anything and read about it.

sorry I meant to type didn’t turn over

The battery seemed to be OK but there was a bunch of corrosion on the +ve terminal. I cleaned it off (not well) but that didn’t seem to help.

Well … Let’s get that battery cable totally clean and tight (both ends) as well as the other battery cable and see what happens.

After that I would want the battery checked, many places will check it for free.

Do the easy stuff first.