Welcome to the forums! You've already had some great advice from folks far better than me, but will reiterate what Paul said - best way to get better is take lots of pictures! Also be sure to try out different combinations of settings, and then check the EXIF data later to see which worked better.
Can't help you with the Tamron vs Pentax lens question. I have a Sigma APO 70-300, which is said to be lower quality than either of these. But have gotten some fine shots of various "bugs" with it.
If you're willing to deal with manual focus, there are some excellent used old Pentax lenses availble, often much cheaper and sometimes even better quality. I've gotten some great shots with an old Pentax 70-210 lens that was only $65.00