できたらスケッチブックと４Ｂの鉛筆も買ってきておいてね。まだそんなに高い画材はいらないよ。

。。

If you can, grab a sketchbook and a 4B pencil too while you're out. You don't need expensive art supplies yet.

Literal

If-possible sketchbook [and-と] 4B [gen-の] pencil [also-も] buy-[come-and-do-てきておいて] [emph-ね]. Still that-much expensive art-supplies [topic-は] don't-need [emph-よ].

できたら 'if possible' is the conditional form of できる, used as a soft polite request. 買ってきておいて is a triple-auxiliary stack: ~てくる (go get and come back) + ~ておく (do in advance) + ~て (soft imperative), giving 'go get it (and bring it back and have it ready).' まだそんなに~いらない 'you don't need (that much) yet' uses the そんなに + negative frame for 'not particularly / not to that extent.'