simple way to import membership and contributions together
You cannot do this in one import. You have to update one table per import.

You will likely need the API csv Import extension.

First step will be to create your csv files. 
1. If they do not have it already, give each member a unique id number. 
2. If your contact info and contribution are all on the same report, divide those into two csv files. You will need to assign the unique contact id # to each contribution. 

After you have your csv files ready, you can import the contact files and then the contribution files. I recommend that your first import only have 2-3 contacts and then another with 2-3 related contributions. This way you can test it without making a mess of all 300 contacts.

You may need the API csv Import extension to import the Contributions. It depends on your setup and which fields you are importing.

https://wiki.civicrm.org/confluence/display/CRM/Import+Contacts+from+Other+Sources

https://wiki.civicrm.org/confluence/display/CRMDOC/Import+Contributions