Adjusting EQ (that results in making a song sound better, instead of worse) has been about the hardest thing for me.
When I started using a 4-track cassette about 10 years ago I consistantly set both the bass and treble at 3 O'clock (12 O'clock was center, no boost or cut)...I was using my eyes instead of my ears...most of those (mixed-down to cassette) songs came out sounding like they had a layer of fog/dullness laying over them. It took me years to figure out how much damage I was doing to the sound by abusing EQ in the recording stage. If your original songs (on cassette) already sound good that will help a lot in "mastering" your completed songs (after you get them transferred to your computer).
Over the years I have had EQ "aha" moments. I've gone through different stages...I tryed (for a time) fighting the dullness by boosting the highs (this has often been the case with songs that I have transferred from cassette to computer), but then (after many months) I would conclude that the music had a too "shrill" quality (and it still might have the overlying "dull film" quality).
Finally I got the concept (from reading on this site) of cutting frequencies (in addition to boosting)...and (about 2 years ago) I got some decent monitors, have listened to a lot of "professional" music through those monitors, and have developed more of a "gut" (actually "ear") feeling when the EQ is off in a song. Now, when I hear that something is wrong (too dull, or too shrill) I can often make the song sound better (using EQ), and, I expect, that I will continue to have "break-throughs" when it comes to EQ.
I suggest that you spend some time listening to commercial music that sounds good to you (training your ear to what a well-EQed song sounds like), and experimenting with the EQ you have (I've been using a 10-band EQ), getting a feel for how various adjustments affect the sound and have the goal (of course) of improving the overall sound of the song (when EQing a completed/mixed down song, one adjustment might benefit one part of the song, but be detrimental to another, so sometimes a compromise is needed to benefit the song overall). I have found that if the song already sounds good that it may need no more than a tweak in the EQ (a slight adjustment on one or two bands) and possible a hint of FX (maybe a little reverb over a song that was recorded dry).