Some basic rules to making text easily legible is to place the text over an area that doesn't have much going on behind it, and also using at least a 10pt text is preferred, generally 24pt and up for headers, etc.
If your text is placed over a busy background, adding a border/stroke to the text itself can help it stand out some.
As far as pixel size/resolution is concerned, try to work at 300 dpi minimum when you are creating a design in a program like photoshop, then use the "Save for Web and Devices" option when you save out your final .jpg image. Of course you can always work in vector programs like illustrator (or inkscape, which is free), but it depends on your comfort and skill level. Photoshop can do a decent job as long as you follow the rules. =)
Good luck!