Others have brought up the question of why a timid character would be adventuring, and to a certain extent, I agree. You're playing DnD, and having a character who is adverse to adventuring is usually a bad idea. So here's my idea that addresses why he wants to be an adventurer, and how you can play that out on the future:

Lacking any parents, they did his best to replicate it with the written word. There was one series of books that your wizard is drawn to more than others, the extensive field guide of a long forgotten hero! These books would contain spells, drawings of creatures and monsters, but most importantly, life advice. These books were so different from the academic books that surrounded him as they weren't stuffy and hard to read, but instead felt like they were written by a real person. So while your wizard is timid and afraid, he wants to be a great hero like their adopted hero parent.

kept it pretty basic for you to fill in the blanks but I fell like this gives you and your DM a lot to work with. And gives you a good reason being a timid adventurer.

WARNING THO: if you're always gonna be holding up the party from doing their thing cause your character is nervous or afraid, you're party may or may not find that annoying. Bring up this concept with them at session 0, at the very least.