First times tips...

1. Be comfortable with yourself before being intimate with someone else.
2. Communicate that this is your first time - it could make all the difference.
3. Try to get to know the person before being intimate - It helps if the first person you are with is somebody you trust and feel comfortable with.
4. Discuss with them what they like and then decide if these are things you would like to try.
5. Try not to be under the influence of anything - other than your own mind!
6. Relax - Everyone has to start somewhere and you are no exception to the rule.
7. Slow down - Take your time (its not a race to the finish)
   
8. Communicate - If you do don't like something, or if you do, say it!
9. Be realistic - Like all first times, it may not live up to your expectations but you will learn what feels good for you. 
10. Remember, the person you are with is probably just as nervous, despite the show of confidence.

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Books

The Whole Lesbian Sex Book: A Passionate Guide for All of Us

Sexual Intimacy for Women: A Guide for Same Sex Couples

Lesbian Sex Positions: A Guide

Tantric Sex for Women: A Guide for Lesbian, Bi, Hetero, and Solo Lovers

Lesbian Couples: A Guide to Creating Healthy Relationships

True Secrets of Lesbian Desire: Keeping Sex Alive in Long-Term Relationships

Late Bloomers: Awakening to Lesbianism After Forty