If you wish you to install a ramp, you have the option of either buying one, or building one. The choice you make may depend upon your budget and your needs; a ramp designed to merely cross a few steps is much easier to purchase and install, while one designed to span ten feet for use with a heavy power wheelchair must be much more sturdy.
Building a permanent ramp is a very involved and exact affair, but if it something you wish to try by yourself, Lowes has an excellent set of step by step instructions on planning, measuring, and building a ramp.
Any time you will have major renovations to your home--especially those renovations that are designed to making your home a safer living environment for a senior with potential health problems--it is important to make sure that installations are performed properly, and with quality materials. As such, it is recommended to use a licensed, bonded contractor.