Your question is a little vague. What kind of signature are you looking to make?
If it's villagers/town signatures, you can get sprites by googling 'acnl villager sprites' and find backgrounds on google. Just make sure it's good quality. If you pick a low quality picture, your sig could look distorted or grainy. If you're looking for fonts, DaFont is the best. I use Photoshop but if you don't have that, then you can use GIMP or Artweaver as free alternatives. As for tips or tutorials, there's a lot of great header tutorials out there so google them. Every tutorial is different depending on what you're looking to do.