HTV is simply very thin vinyl that comes on a plastic carrier sheet. You simply 'kiss-cut' your design (ie. cut only through the vinyl and not the backing sheet) then 'weed' away the unwanted parts of the vinyl. Your design must be flipped before cutting so that it's the right way round when laid on your garment for pressing. No sprays are needed, the vinyl adheres to the garment fibres under heat and pressure. The results are long-lasting and can be washed many times without any ill effects.
Note that there are basically two main types of vinyl: HTV for garments, and Sign Vinyl, which has adhesive on the back and is used for signs, window signs and vehicle wraps, so be careful you buy the correct type.
You can buy vinyl from many outlets, but as a guide to what's available check out MDP Supplies here -
https://www.mdpsupplies.co.uk/Default.asp Make sure you look in the 'Clothing Vinyl' section for HTV.
Generally, vinyl comes in solid colours or patterns, so is no good for photographic designs or designs with smooth gradients, but different colours can be 'layered' on top of each other for effect (make sure use the same types of vinyl when layering).
Once you have a cutter this will open up a world of possibilities. Vinyl is relatively cheap and easy to work with and produces professional results. Of course, depending on the complexity of your designs, weeding can become a bit of a chore if you have many garments to do, but it is very effective for small to medium runs and one-off's. Vinyl has a 'hand,' or 'feel' to it, especially on full chest designs so for something with no hand you'd need to consider sublimation, which is a whole other ball game, and you'll find plenty of threads a about getting started in here.