It's easy enough in Photoshop to go to Image > Image Size and put in larger numbers. However, your image editing program has to interpolate and do it's best to guess at making the image larger by filling in blanks.
If at all possible the best bet is to rescan the image at a larger resolution. Instead of scanning in at 72dpi try150dpi for example. If you can re-shoot the photo this could give you a larger file too. However, this isn't always possible and you have to work with the image you have.
Here are a couple of tricks that might make your results a little better.
In Photoshop, go to Image > Image Size. From the drop-down menu provided select "Bicubic Smoother (best for enlargement).
The next thing to do is to increase the size by small increments instead of all at once. For example try increasing the size by 10% twice than by 20% once. This seems to get better results. And of course, you'll only be able to increase the file so much before it's totally unusable.
If you don't like the results Photoshop gives you for increasing the image size there are a few other options to try.
Try upsizing in Photoshop and if you don't like the results, it doesn't cost a thing to try reshade.
I downloaded a free trial version and upsized in Photoshop then tried upsizing again in Perfect Resize. I was surprised and pleased at the results shown here.
There is a noticable difference in the quality of the photograph on the left done with Perfect Resize compared with the photograph on the right that I did with Photoshop.
If you decide you like the software after testing the trial version, it can be purchased for only $99.
Try these suggestions the next time you need to make a photograph larger. One of these should get you decent results.