For me the hardest part is getting the tracks down, and then mixing and mastering them. The best thing you can do is sit with an instrument, find a chord progression you like, make a melody to that progression and write lyrics over that.
When Im starting with just lyrics, I come up with a beat, a speed I like. Then each verse starts and ends on the beat, longer words can use semi beats but its critical you start and end on the beat. Say its a 1-2-3-4 beat, it starts on 1 and ends on 4.
From there, come up with a melody and a key. You may need to edit the words to fit the melody, but keep that beat on the 1-4. Once you have a melody you can go from there. Personally start with the music, record it into whatever software you have and put it on loop, find a verse sound, a chorus sound, and a bridge sound.
When you have those sections the melody to the music comes simple. Flow in a A B format, A progression, B progression, C for chorus, D for bridge. When you come to write a bridge, I like to do a stripped down feel, just bass and drums for example.
And you do need to invest a little, the more you put in, the more than comes out in the end. But you can get cheap equipment, but you will be bottlenecking the end result a little.