Start with reading and understanding wallet v3 code here:
https://github.com/ton-blockchain/ton/blob/master/crypto/smartcont/wallet3-code.fc
Then maybe check wallet v4:
https://github.com/ton-blockchain/wallet-contract/blob/main/func/wallet-v4-code.fc
Then you may read NFT Collection and Item implementation:
https://github.com/ton-blockchain/token-contract/blob/main/nft/nft-collection.fc
https://github.com/ton-blockchain/token-contract/blob/main/nft/nft-collection.fc
Finally, Jetton minter and wallet are great starting points too:
https://github.com/ton-blockchain/token-contract/blob/main/ft/jetton-minter.fc
https://github.com/ton-blockchain/token-contract/blob/main/ft/jetton-wallet.fc