Just read the first post of this thread carefully and ask more specific questions if you still have some.

Short:
1. Download the wallet and let it sync
2. Get at least 10 BTX (Claim your BTX if you've had some Bitcoin in april on your own address with a ratio of 1:1 and/or buy yourself some at cryptopia for example)
3. Transfer your BTX to your own wallet
4. Wait for monday to get your 3% - as long as this airdrop continues like that

Once your (at least) 10 BTC are on your own address, the wallet can be offline as well - you still get your 3%

Edit:
Or mine yourself some BTX e.g. with a pool (but for that you have to read even more)