Yes, using a melted ghetto rigged 'syringe' you fashioned would be incredibly irresponsible and totally out of line with harm reduction, and sterility. I don't know where you live or what the laws are, but do they sell syringes over the counter in pharmacies? If not, there is bound to be a syringe exchange somewhere in your area.. Otherwise, you can order syringes via the internet at a very low cost.. You don't want to re-use syinges, even twice.
Try the pharmacy. Just make sure you know what you are asking for so you don't come off as a junky. But if you are diabetic, you can bring you insulin as proof and it shouldn't be a problem. Just ask for, for example - "A box of 30 gauge, short tipped 1cc insulin syringes, please".
But if you are serious about shooting up, PLEASE do all the research necessary, and then read it again. Then again. Here on bluelight, we have an injection mega thread, which you can find via the search engine. This should help you greatly. Also there's a website called herion helper that is very informative. But if you are dead set on injecting, it is imperative you know EVERYTHING you need to know about how to inject properly, safely, and cleanly. Trying to learn how to shoot up and not being as safe as possible is a recipe for disaster.
Injecting is NOT to be taken lightly. It is a very slippery slope and I hope you are very aware of the pros/cons of doing so. If I could go back a few years and stop myself from learning how to shoot up, I DEFINITELY would.. It's been nothing but trouble for me in my life, and most others can attest to this as well.
Also, do you know anyone in real life who shoots up? If so, it would be a good idea to have them show you how to do it. This will be much easier than trying to teach yourself, since the internet can only show you so much. It will probably take some trial and error.