Evening.  New player to D&D here, started getting interested after the Wildemount expansion released.  I had an idea for a character; 

Taking advantage of Wildemount's new races, I decided to create a green Draconblood Dragonborn wizard; however, I was kinda needing some help.  First off, could someone explain the Shapechange spell?  I was kinda hoping to use the ability to turn into different animals for a role-play situation, but I couldn't figure out the specifics of the spell; like, is what you can turn into determined by size and such?  And can you still cast spells while in your alternate form?  

The second, and a bit more serious, question; can you make a Dragonslayer Dragonborn Wizard build?  I've heard of the difficulties slaying the big lizards, and I would like some advice.  

Thank you for your time.  

There is no spell called “Shapeshift” so I don’t know how to help you.

By Shapeshift, did you mean Polymorph?

If you did, you migh tfind this useful: https://media.wizards.com/2015/downloads/dnd/DnD_MonstersByType_1.0.pdf

Basically when you cast the spell you can change yourself or another creature into any creature from the "Beast" section of that list as long as the CR (shorthand for challenge rating) of the new form is lower than your level if you target yourself or the targets level or CR if you target another creature.

So if you were a level 7 wizard and cast polymorph your could turn yourself into a Giant ape or a Giant Crocodile, if you cast it on a creature, for instance a Umber Hulk (using a Crit Role example) then you could change it into a Snail and then throw it over a waterfall.

Important thing to remeber is that if you deal enough damage to kill the form you shifted it into then it reverts back to its normal form.

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As for being a Dragonslayer...that might be more of a quest to seek out various magical items (Dragon Slayer Swords, Dragon Scale Armour etc) but you can pick spells to buff your self and party to resist Dragon attacks (Absorb Elements & Protection from Energy) or to try and debuff Dragons (Earthbind & Slow) or just deal as much damage as possible (such as Disintergrate), but it all depends on your play style/Wizard subclass, whether you are going to target one particualr type of dragon or try to slay them all and whether Dragons are a big thing in your DM's campaign.

Correction; I meant Shapechange.  Sorry about the confusion; tried to do some research, but I couldn't find anything conclusive.  

Thanks for the tips on dragonslaying!  I'll have to run them by the GM.  

Shapechange is a 9th-level spell. To cast that you will need a 17th level spellcaster. Most campaigns never make it that far, and the ones that do take years. I would worry about Shapechange as decision making consideration at this point. By the time you can cast it, you will be able to understand it.

Ah, I see.  Well, that's a bit disappointing; still, thanks for the info.  

If you want to change into animals, you might be more interested in a Druid. They can do it from level 2.