Lofts usually have some sort of hatch for access, but this often be just a gap in the roof with a rectangular section of plywood covering it. But if you regularly need to store or access items in the loft, then you will probably get fed up with fetching the ladders out of the shed every time and struggling to remove and refit that piece of plywood. You should really think about getting a permanent ladder fixed, this could be done leaving the existing hatch as is, by adapting the hatch cover to hinge downwards and then fit a ladder, or by replacing the hatch by fitting an integral hatch and ladder assembly.
A proper loft ladder makes accessing the loft space so much easier and safer too. The ladder that you choose will depend on your preference, the space available and how often you need to use it. Loft ladders range from basic aluminum folding ladders through to concertina type ladders to save space, expensive hardwood folding ladders, or even fully automatic electric ladders custom designed for your loft-space. However, you should note that loft ladders are only suitable for using your loft as storage space. If you plan on converting your loft-space into a habitable room, you will need a permanent staircase to comply with current building regulations.
There are only two main types of loft ladder, either sliding and folding, each of which can be made of metal (usually aluminum) or timber. Sliding ladders are the easiest to fit and usually come as two or three sections and are abut if not they can easily be modified to fit one. Many loft ladders come with a kit included so you can convert the existing hatch into a hinged hatch. The cheaper sliding loft ladders are made of aluminum and designed for occasional usage. They are much noisier than the more expensive timber versions but do the job for a low price.
The three main types of folding ladders are concertina, telescopic and simple folding. Concertina ladders are excellent for use in tight spaces as they require virtually no clearance in your loft and also have small landing requirements. But concertina ladders are complex to build and they require so much engineering they cannot be manufactured cheaply. Telescopic ladders have the same space saving qualities but with a more minimalist and modern design. Telescopic ladders look fantastic and can fit in really tight spaces, plus the simpler design means lower costs when compared to concertina type loft ladders. However, the simple folding ladders are cheapest of all, but they do require enough clearance to fold up in the loft, plus enough space to fold out on the landing.
If thinking about a DIY loft hatch/ladder installation, it really all depends on your carpentry skills. You obviously need a loft ladder that is safe to use, which does not compromise the structural integrity of the ceiling. If you are confident you can do this, then you could do it yourself. However, there are many benefits to using a professional, including a professional finish and quicker installation.