The best advice I can give you is 1) know what you are applying for 2) know if it's something you can see yourself doing for the rest of your life, and 3) go in there with the confidence that if they advertise hiring for $14-20 an hour you get hired at $20 an hour.
I will say though, fresh out of a high school or college trade school degree with no field experience will limit your starting pay, but that doesn't mean you can't ask for a raise or reassessment in 90 days.
If you can, practice fabrication of parts, not just welding 2 plates together over and over. Weld in multiple processes and positions. At my job I MIG (GMAW), TIG (TGAW), and stick (SMAW) weld aluminum, stainless, mild steel and brass in all positions pretty much daily. I also oxyacetylene weld, but that's an out dated process. I have multiple certifications in all of them through my employer. Your school degree or certificate means you know how to weld, but each individual employer actually has to certify you for insurance purposes. The only welding certifications that have any weight out of school would be a national welding cert like AWS, or similar programs.