75/25 split with you mentor for the first 5 deals is fair. Starting out, you really need someone there to guide you. Make sure you have a good mentor though.
You current financial situation will determine how you start. If you are cash strapped, started with being a buyer's agent. Buyer's agent has lower cost, but is not scalable.
If you are not cash strapped, start with lead gen(for seller leads), there are just too many ways to do that, direct mailing, google, facebook etc
Either way here is a list of things you will need
1. like @David Hunter said, you will need a lead bait, don't do anything without a lead bait. The effectiveness of your lead gen will depended on two thing: your list and your lead bait.
2. Started a contact database - your long term success in real estate is highly correlated to the size of your contact database. If you only focus on one metrics, focus on growing the size of your contact database.
3. Follow up with your contact regularly - it's just a matter of calling or email them once a while. There are tools to help you do that - contactually works really well for most of agents I talked to.
4. A website - this is your home base. This is where you turn yourself from a real estate sales person into real estate consultant. All outbound marketing effort should drive traffic to your website.
5. Ask other agents if you can host open house for them - from all the agents I have talked to, this is the best and most efficient way to get buyer leads. You can offer referral fee to the listing agent.
Being an agent is not an easy business. 50% fail, 40% barely floating, remaining 10% is taking most of the market shares. You goal will need to be that 10%.
Hope this helps.