although this is a very basic and general question, I can't resist answering that a simple search on google for "sequence alignment" will return 2 entries on wikipedia (sequence alignment itself and sequence alignment software) that are of great help for beginners on sequence alignment field.
as a future advice, you don't need to invest your time posting basic questions on an advanced forum, since any search engine will take you to the best places to start reading about what you need.
Although I love all the tools mentioned here, I rate MUSCLE as the best and is my personal favourite. If you have a lot of 3D structures available for your gene, you could use PROMALS3D which uses the structural information as well in aligning the sequences which is particularly useful at times.
Don't forget to correct the sequence alignment by eye , as no matter what tool you use you never get a perfect alignment.