1. Use a capo.

2. Make sure, however, that it is the right kind of capo for YOUR specific guitar(s), because capos are not all the same, just as guitars are not all the same. Some capos will be fairly useless on your guitar. Others will fit it like a dream.

3. You must position the capo correctly for a good result. If you do, the guitar will sound just as good as it does with open strings.

4. Get a well qualified guitar player at a good music store to demonstrate all of the above to you and show you exactly how to do it and why it's done that way.

(and I am just repeating what many others have said earlier in this thread)